fix(docs): update Open Graph and Twitter meta tags for improved sharing

[julioest]

This PR includs large preview cards for Mr.Docs links on X so shared URLs look better.

• Adds minimal twitter:* + OG fallbacks
• Sets twitter:site=@cppalliance; creator optional/overridable
• Uses absolute HTTPS og:image
• Keeps robots open so Twitterbot can fetch page/image
• X caches ~7 days; refresh by tweeting a fresh URL param

Design Preview

@rbbeeston Lmk if we're okay with this design, I can make more variations - just lmk!

[cppalliance-bot]

An automated preview of the documentation is available at https://1075.mrdocs.prtest2.cppalliance.org/index.html

If more commits are pushed to the pull request, the docs will rebuild at the same URL.

2025-10-17 20:05:44 UTC

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.48%. Comparing base (13e9230) to head (0089138).
⚠️ Report is 1 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1075      +/-   ##
===========================================
- Coverage    85.48%   85.48%   -0.01%     
===========================================
  Files          307      307              
  Lines        23851    23850       -1     
===========================================
- Hits         20388    20387       -1     
  Misses        3463     3463              

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[rbbeeston]

That looks fantastic!

[alandefreitas]

Wow!

[alandefreitas]

More than approved by me!