feat: important dates page ids

[holloway]

Provide a custom IETFn id rather than the auto generated one autoid-n.

fixes #8066.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.94%. Comparing base (c7f6bde) to head (6d9ee01).
Report is 183 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8175      +/-   ##
==========================================
+ Coverage   88.78%   88.94%   +0.15%     
==========================================
  Files         296      303       +7     
  Lines       41320    41260      -60     
==========================================
+ Hits        36687    36698      +11     
+ Misses       4633     4562      -71     

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