Sandcastle cross links

[jjspace]

Description

• Add cross links between old and new sandcastle for the "soft release" and help gain traction
  • Removes the "Feedback" panel added in Add feedback section to current sandcastle #12641 from old sandcastle
  • I experimented with trying to "preserve" the current sandcastle across the environments. Going from Old to New worked nicely. Going from New to Old does not because the structure changed. I opted to not do it in either direction to avoid confusion around why only one way works.
• Update CI to only run in the production cesium.com branch to match our release process

Note this relies on PR #12832 for the popovers and should be merged after that one

Issue number and link

Part of #12566

Testing plan

• Open current sandcastle
• Make sure the banner looks good and links to dev-sandcastle.cesium.com
• Open new sandcastle
• Make sure the banner looks good and links to sandcastle.cesium.com. Note this banner is intentionally more subtle
• Make sure the "Feedback" popover in the top right looks good (issue link tbd)

Author checklist

• I have submitted a Contributor License Agreement
• I have added my name to CONTRIBUTORS.md
• I have updated CHANGES.md with a short summary of my change
• I have added or updated unit tests to ensure consistent code coverage
• I have updated the inline documentation, and included code examples where relevant
• I have performed a self-review of my code

[github-actions]

Thank you for the pull request, @jjspace!

✅ We can confirm we have a CLA on file for you.

[ggetz]

I, of course, have wording suggestions. 🙂 Happy to push them up myself, but I did want to check on the github link.

[jjspace]

@ggetz updated, now referencing issue #12857