[docsy] Post-LFX T2 mentorship site work

[chalin]

• Ensure grant info appears on the site and in footer:
  • [docsy] Add About page to hold project and grant info #86
  • [docsy] About section rework: add headers #89
• Ensure base specs page path is /specs rather than /specifications/specs - [docsy] Reworked Spec page, including wildcard redirects #88
• Add support for external link checking #82
• [docsy] New IA #93
• [docsy] Recover security audits page #94
• [docsy] Adjust links to spec, community contrib page, repo src #101
• [docsy] Spec page: make link to latest a button, tweak external link rules #104
• [docsy] Enable site search #91 --> will be worked on later
  • [docsy] Configure Google Custom Search Engine but leave it disabled #92
• Pre-LFX website: create a branch, and enable branch deploys over it, so we can view the pre-LFX version. Done:
  https://pre-lfx-2024--theupdateframework.netlify.app
• Remove docsy from the branch deploy list
• Submit PR from docsy branch - New and improved Docsy-based version of the website  #105
• Drop the github_branch setting, so that we fall back to main - Adjustments for page-meta links #106

Resources:

• Pre-LFX website version: https://pre-lfx-2024--theupdateframework.netlify.app
• @lukpueh's main comments about an earlier version of the Docsy-based version of the website: Feedback on Docs and community sections on Docsy theme Dindihub/docsy-tuf.io#1 (review)