chore: add TemplateMark dingus retirement page

[Rishabh060105]

Closes #655

Adds a repo-tracked retirement page for the legacy TemplateMark dingus so the stale v0.16.2 demo can be replaced with a static archival notice instead of restoring removed packages.

Changes

• add support/templatemark-dingus-retired/ with a static replacement page and stylesheet for the retired dingus site
• add deployment handoff notes and an issue-close comment template for the Netlify site owner who will replace the live demo

Flags

• this PR does not itself redeploy https://templatemark-dingus.netlify.app/; the current Netlify account in this environment does not have access to the existing site
• no unit or integration tests were added because this change is a static retirement page; the page was verified locally in the in-app browser

Screenshots or Video

Related Issues

• Issue TemplateMark dingus preview is blank: browser bundle pulls browser-unsafe markdown-pdf code #655

Author Checklist

• Ensure you provide a DCO sign-off for your commits using the --signoff option of git commit.
• Vital features and changes captured in unit and/or integration tests
• Commits messages follow AP format
• Extend the documentation, if necessary
• Merging to main from Rishabh060105:Rishabh060105/retire-templatemark-dingus
  index.html