fix: correct the rfc-index example (See Also is STD/BCP/FYI, not RFC)

[youdie006]

The example at the top of the generated rfc-index.txt (from ietf/templates/sync/rfc-index.txt) shows RFC 1129 with a (Also RFC1119) See Also:

  1129 Internet Time Synchronization: The Network Time Protocol. D.L.
       Mills. October 1989. (Format: TXT, PS, PDF, HTML) (Also RFC1119)
       (Status: INFORMATIONAL) (DOI: 10.17487/RFC1129)

But the file's own "Key to citations" states that (Also FYI ##) / (Also STD ##) / (Also BCP ##) only give an equivalent FYI/STD/BCP number — a See Also never points at another RFC — and the actual RFC 1129 entry has no See Also. So (Also RFC1119) is not a valid example.

This replaces it with the RFC 9915 example suggested in #11280, which carries a genuine (Also STD102):

  9915 Dynamic Host Configuration Protocol for IPv6 (DHCPv6). T. Mrugalski,
       B. Volz, M. Richardson, S. Jiang, T. Winters. January 2026. (Format:
       HTML, TXT, PDF, XML) (Obsoletes RFC8415) (Also STD102) (Status:
       INTERNET STANDARD) (DOI: 10.17487/RFC9915)

Continuation lines are indented to align under the title, matching the surrounding template style. One file, docs/template only.

Closes #11280.

---

Disclosure: prepared with AI assistance; I reviewed the change against the file's Key to citations and the suggested fix in the issue.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.58%. Comparing base (306f479) to head (5f429d0).
⚠️ Report is 19 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #11320   +/-   ##
=======================================
  Coverage   88.58%   88.58%           
=======================================
  Files         333      333           
  Lines       44662    44670    +8     
=======================================
+ Hits        39565    39573    +8     
  Misses       5097     5097           

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