Code Quality Enhancements and Docs

[JimBobSquarePants]

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
• I have verified that I am following matches the existing coding patterns and practice as demonstrated in the repository. These follow strict Stylecop rules 👮.
• I have provided test coverage for my change (where applicable)

Description

• Adds bounds checks to Format4SubTable. Fixes IndexOutOfRangeException in Format4SubTable.TryGetGlyphId with malformed CMap table #510
• Renames culture aware Add overloads to AddWithCulture in IFontCollection to make them more obvious and prevent consumer build warnings.
• Adds comprehensive XML documentation to all tables and methods.

[codecov]

Codecov Report

❌ Patch coverage is 47.91667% with 50 lines in your changes missing coverage. Please review.
✅ Project coverage is 82%. Comparing base (48b8ab1) to head (4798947).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
src/SixLabors.Fonts/Tables/General/Colr/Paint.cs	37%	48 Missing ⚠️
...abors.Fonts/Tables/General/CMap/Format4SubTable.cs	83%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #511   +/-   ##
=====================================
  Coverage     82%     82%           
=====================================
  Files        305     305           
  Lines      21520   21528    +8     
  Branches    3161    3164    +3     
=====================================
+ Hits       17797   17806    +9     
+ Misses      2916    2911    -5     
- Partials     807     811    +4     

Flag	Coverage Δ
unittests	82% <47%> (+<1%)	⬆️

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