Further environment (documentation) improvements

[Darwinkel]

This is a follow-up ticket to #1517. Remaining work:

• Remove and clean up old documentation about environment settings
• Implement Pydantic in Rocky settings such that documentation and validation can be automated for Rocky as well #1830
• Perhaps some aesthetic changes (maybe a nice table instead of a list of variables?)
  • Current look is sufficient imho
• Prepare upgrade/migration instructions for the release notes and the community chats
  • Double-check required migrations/instructions for the Debian packages and environments
  • Fix configuration issues upgrading Debian installs #1636
  • https://docs.openkat.nl/release_notes/1.11.html
• Update community scripts (Add community install/update scripts #1309)
  • Update install script with new env vars #1655
• Update example code in the docs
• Make sure Mula's documentation is correctly updated as well (Update scheduler documentation #1476)

[Darwinkel]

Closing for now in favour of follow-up ticket: #1830