Backport #28302 by @yp05327

Close #28287

## How to test it in local
convert Makefile L34 into:
```
cd .tmp/upstream-docs && git clean -f && git reset --hard && git fetch origin pull/28302/head:pr28302 && git switch pr28302
```

Co-authored-by: yp05327 <576951401@qq.com>

This PR cleans up the docs in a way to make them simpler to ingest by
our [docs repo](https://gitea.com/gitea/gitea-docusaurus).

1. It includes all of the sed invocations our ingestion did, removing
the need to do it at build time.
2. It replaces the shortcode variable replacement method with
`@variable@` style, simply for easier sed invocations when required.
3. It removes unused files and moves the docs up a level as cleanup.

---------

Signed-off-by: jolheiser <john.olheiser@gmail.com>

This was intended to be a small followup for
https://github.com/go-gitea/gitea/pull/23712, but...here we are.

1. Our docs currently use `slug` as the entire URL, which makes
refactoring tricky (see https://github.com/go-gitea/gitea/pull/23712).
Instead, this PR attempts to make future refactoring easier by using
slugs as an extension of the section. (Hugo terminology)
- What the above boils down to is this PR attempts to use directory
organization as URL management. e.g. `usage/comparison.en-us.md` ->
`en-us/usage/comparison/`, `usage/packages/overview.en-us.md` ->
`en-us/usage/packages/overview/`
- Technically we could even remove `slug`, as Hugo defaults to using
filename, however at least with this PR it means `slug` only needs to be
the name for the **current file** rather than an entire URL
2. This PR adds appropriate aliases (redirects) for pages, so anything
on the internet that links to our docs should hopefully not break.
3. A minor nit I've had for a while, renaming `seek-help` to `support`.
It's a minor thing, but `seek-help` has a strange connotation to it.
4. The commits are split such that you can review the first which is the
"actual" change, and the second is added redirects so that the first
doesn't break links elsewhere.

---------

Signed-off-by: jolheiser <john.olheiser@gmail.com>

- **Installation**: includes how to install Gitea and related other
tools, also includes upgrade Gitea
- **Administration**: includes how to configure Gitea, customize Gitea
and manage Gitea instance out of Gitea admin UI
- **Usage**: includes how to use Gitea's functionalities. A sub
documentation is about packages, in future we could also include CI/CD
and others.
- **Development**: includes how to integrate with Gitea's API, how to
develop new features within Gitea
- **Contributing**: includes how to contribute code to Gitea
repositories.

After this is merged, I think we can have a sub-documentation of `Usage`
part named `Actions` to describe how to use Gitea actions

---------

Co-authored-by: John Olheiser <john.olheiser@gmail.com>

Followup to #21071 
Thanks for the catch @wxiaoguang 

[WRT](https://github.com/go-gitea/gitea/pull/21071#discussion_r962706673)

Ref #21071

* Add documentation about pagination of the API

* Update docs/content/doc/developers/api-usage.en-us.md

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

Add `markdownlint` linter and fix issues. Config is based on the one
from electron's repo with a few rules relaxed.

* `git` -> `Git`
* `Github` and `github` -> `GitHub`
* `crowdin` -> `Crowdin`
* `git-lfs` -> `Git LFS`
* `githooks`, `git hooks`, `git-hooks` -> `Git Hooks`
* `discord` -> `Discord`
* `2fa` -> `2FA`
* `gitlab` and `Gitlab` -> `GitLab`
* `web hook` -> `webhook`
* `linux` -> `Linux`
* `sqlite` -> `SQLite`
* `MYSQL` and `mysql` -> `MySQL`
* rename refs to `master` branch -> `main`
* Fix English grammar

Move the token API discussion into a common section discussing the
generation and listing of the tokens.  Add a note on the display of
the sha1 during creation and listing.

Co-authored-by: Norwin <noerw@users.noreply.github.com>

* Added OpenAPI document link to usage

The OpenAPI document at /api/swagger.v1.json needs an obvious reference.  Sadly, I am English monolingual, so someone else is going to have to do the other languages.  In the mean time, this PR should help anyone looking for the file.

* Update docs/content/doc/developers/api-usage.en-us.md

Co-authored-by: a101211279 <1012112796@qq.com>

Co-authored-by: techknowlogick <matti@mdranta.net>
Co-authored-by: a101211279 <1012112796@qq.com>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>

Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: Lauris BH <lauris@nix.lv>

* Redo #13836 with new shortcode
(https://gitea.com/gitea/theme/pulls/90)

* add Api Usage

Co-authored-by: techknowlogick <techknowlogick@gitea.io>

* First pass

Signed-off-by: jolheiser <john.olheiser@gmail.com>

* More changes

Signed-off-by: jolheiser <john.olheiser@gmail.com>

* Redirects

Signed-off-by: jolheiser <john.olheiser@gmail.com>

* add sdk links to docs

* Update docs/content/doc/advanced/api-usage.en-us.md

Co-authored-by: John Olheiser <john.olheiser@gmail.com>

Co-authored-by: Markus <git+markus@obermui.de>
Co-authored-by: John Olheiser <john.olheiser@gmail.com>

It's(swagger api link) mentioned vaguely in the FAQ but IMHO missing from API usage page.

* Add oauth2 documentation

Signed-off-by: Jonas Franz <info@jonasfranz.software>

* Apply suggestions from code review

Co-Authored-By: jonasfranz <info@jonasfranz.software>

* Update docs/content/doc/advanced/oauth2-provider.md

Co-Authored-By: jonasfranz <info@jonasfranz.software>

* Update oauth2-provider.md

Updates documentation to account for the renamed ENABLE_SWAGGER_ENDPOINT from  #3572 (commit a74426d631)

Signed-off-by: Peter Colberg <peter@colberg.org>

* add api user guides in doc

* update user-guides api page

* fix typo: user guides -> user guide

* move api-usage page under advanced category

* flesh out API usage docs

* Build on work by @tungsheng

* Address issues raised in #4037, #3673, and #4243

* Close #4247

Signed-off-by: Steve Traugott <stevegt@t7a.org>