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/.gitpod.yml

@@ -0,0 +1,15 @@
+ports:
+  - port: 8080
+    onOpen: open-browser
+
+tasks:
+  - name: Docs
+    init: make build
+    command: make run
+  - name: Notice
+    command: |
+      clear
+      echo "Once the docs server is running, go to the following URL to view the docs:
+
+      $(gp url 8080)"
+    openMode: split-right