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/.gitignore

@@ -0,0 +1,17 @@
+# Mac OSX
+.DS_Store
+
+# VIM
+*.swp
+
+# Build
+v4/html/index.html
+*mattermost-openapi-v3.yaml
+*mattermost-openapi-v4.yaml
+.npminstall
+node_modules
+playbooks/*.yaml
+!playbooks/tags.yaml
+
+# IDEs
+.idea/