Short-Live Environments for PR Reviews

[akyriako]

• [x] Add Demo Environment (https://github.com/akyriako/docusaurus-play)
• [x] Scaffold Release Workflow
• [x] Find a fix for dynamic baseUrl

Move all to docs-next repo:

• [x] Adjust workflow names
• [x] Add new environment pull-requests-preview
• [x] Add variables in every environment DOCS_NEXT_ORG, DOCS_NEXT_HOST, DOCUSAURUS_BASE_URL
• [x] Set variables in env segment of the Build App step of every workflow
• [x] Add dynamic Url values in docusaurus.config.ts