Move API Reference (#23777)

* merge mattermost-api-reference unchanged

* api: update repostiory paths

* api: drop GitPod for api (for now)

* api: improved node_modules target

* api: relocate GitHub actions to root

* Update .github/workflows/api.yml

Co-authored-by: Antonis Stamatiou <stamatiou.antonis@gmail.com>

* fix cache-dependency-path

* adopt node-version-file

* pin versions for uses

* tidy steps/runs

* api/.gitpod.yml: tidy

* api: rm now unused .gitlab-ci.yml

---------

Co-authored-by: Antonis Stamatiou <stamatiou.antonis@gmail.com>

api/CONTRIBUTING.md

@@ -0,0 +1,13 @@
+## Contributing
+
+We're accepting pull requests! Specifically we're looking for documenation on routes defined [here](../server/channels/api4).
+
+All the documentation is written in YAML and found in the [source](v4/source) directory.
+
+* When adding a new route, please add it to the correct file. For example, a channel route will go in [channels.yaml](v4/source/channels.yaml).
+* To add a new tag, please do so in [introduction.yaml](v4/source/introduction.yaml)
+* Definitions should be added to [definitions.yaml](v4/source/definitions.yaml)
+
+There is no strict style guide but please try to follow the example of the existing documentation.
+
+To build the full YAML, run `make build` and it will be output to `html/static/mattermost-openapi.yaml`. To check for syntax, you can copy the contents of that into http://editor.swagger.io/ or you can look into using a commandline or ESLint-based syntax checker.