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

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

api/.spectral.yaml

@@ -0,0 +1,14 @@
+extends: '@ibm-cloud/openapi-ruleset'
+rules:
+  operation-tags: off
+  property-case-convention: off
+  request-body-object: off
+  operation-operationId: error
+  operation-summary: warn
+  array-responses: off
+  parameter-description: error
+  parameter-description: off
+  parameter-case-convention: info
+  no-$ref-siblings: off
+  enum-case-convention: off
+  path-segment-case-convention: info