fix: only allow first h1 to be the title

[yufeih]

• Extracts title from the first H1 for conceptual documents, which was previously from H1/H2/H3.
• Drop IsUserDefinedTitle.
• Consolidate conceptual build steps into one.

#3124

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (0a3bd2c) 71.35% compared to head (fbf5e8e) 71.33%.

Files	Patch %	Lines
.../Docfx.Build/Conceptual/BuildConceptualDocument.cs	96.29%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9474      +/-   ##
==========================================
- Coverage   71.35%   71.33%   -0.02%     
==========================================
  Files         561      559       -2     
  Lines       24005    23994      -11     
  Branches     4177     4174       -3     
==========================================
- Hits        17128    17117      -11     
  Misses       5782     5782              
  Partials     1095     1095              

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