Build docs preview and production builds in GitHub CI

[m-vdb]

Proposed changes

• 2 workflow changes:
  • On pull requests, build the docs, upload a preview to Netlify and add a comment to the PR with a link to the preview (only run the workflow if there were docs changes to avoid spamming devs). It uses the PR number for the deploy alias, so that it's deterministic.
  • On the documentation branch, build the docs and upload a production build to Netlify. Upon failure, send a message to Slack.
• Secrets have been updated in the Respository settings

After merging

• Manually disable deploys on Netlify
• The changes should be automatically added to the documentation branch. If not (for whatever reason), investigate and manually port the changes there;
• Do similar changes to the SDK and Rasa X
• Update the doc on documentation workflows in Notion
• Prepare rollback plan

How to rollback

• Comment the 2 workflow jobs introduced in this PR (not the whole workflow file). This needs to happen on the main branch, and should be automatically propagated to the documentation branch. If not, for some reason, do it manually.
• Go to https://app.netlify.com/sites/rasahq-docs-rasa-v2/deploys and click "Active Builds"

[github-actions]

🚀 A preview of the docs have been deployed at the following URL: https://10568--rasahq-docs-rasa-v2.netlify.app/docs/rasa