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/README.md

@@ -0,0 +1,28 @@
+# Mattermost API Documentation
+
+This repository holds the API reference available at [https://api.mattermost.com](https://api.mattermost.com).
+
+The Mattermost API reference uses the [OpenAPI standard](https://openapis.org/) and the [ReDoc document generator](https://github.com/Rebilly/ReDoc).
+
+All documentation is available under the terms of a [Creative Commons License](https://creativecommons.org/licenses/by-nc-sa/3.0/).
+
+## Contributing
+
+We're accepting pull requests! See something that could be documented better or is missing documentation? Make a PR and we'll gladly accept it.
+
+All the documentation is written in YAML and found in the [v4/source](v4/source) directories. APIv4 documentation is in the [v4 directory](v4).
+APIs for [Playbooks](https://github.com/mattermost/mattermost-plugin-playbooks) are retrieved from GitHub at build time and integrated into the final YAML file.
+
+* 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 `v4/html/static/mattermost-openapi-v4.yaml`. This will also check syntax using [swagger-cli](https://github.com/APIDevTools/swagger-cli).
+
+To test locally, run `make build`, `make run` and navigate to `http://127.0.0.1:8080`. For any updates to the source files, re-run the same commands.
+
+## Deployment
+
+Deployment is handled automatically by our Jenkins CLI machine. When a pull request is merged it will automatically be deployed to [https://api.mattermost.com](https://api.mattermost.com).