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>