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.

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

As title. And needs to backport to 1.19

This PR also adjusts the weights.

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Co-authored-by: delvh <dev.lh@web.de>

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.

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

For Docusaurus, it needs a meta data named `sidebar_position`, so we
copy all `weight` under `menu/sidebar` as top meta key.

- **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

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

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

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

* import docs into main repository

Signed-off-by: Matti Ranta <matti@mdranta.net>