Update upstream BookStack source as project moved from github to codeberg

[cdrfun]

• I have read the contributing guideline and understand that I have made the correct modifications

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Description:

The Bookstack project moved away from GitHub.
This PR updates the BookStack build pipeline and documentation, so the upstream project source uses Codeberg instead of GitHub. The change covers release discovery, download URLs, documentation references, and the repository changelog. I spoke to @ssddanbrown about some details, and he said there could be a 5-minute delay before the tarball is updated. Therefore, I added a configurable delay, currently set to 6 minutes.

Benefits of this PR and context:

• Removes dependency on the GitHub BookStack release API.
• Aligns release detection with the actual upstream project hosting location.
• Uses Codeberg RSS for automated version parsing in build flows.
• Updates documentation links and changelog entries to the new upstream source.

How Has This Been Tested?

• Local build of Dockerfile with explicit BOOKSTACK_RELEASE
• Local build of Dockerfile without BOOKSTACK_RELEASE to verify automatic RSS-based version detection
• Local and GitHub Workflow tests to verify trigger (in test branch, not included in this PR)

Source / References:

• https://www.bookstackapp.com/blog/project-migrated-to-codeberg/
• https://codeberg.org/bookstack/bookstack

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[cdrfun]

Closed because of #292