Fix config lookup: use 'is None' instead of 'or default', add get_config_flex and html_extra_path support

[timkpaine]

• Add get_config_flex() to accept both hyphens and underscores in config keys
• Replace 'or default' pattern with 'is None' check in configuration_args loop to avoid overwriting intentional falsy values (False, [], 0, etc.)
• Switch use_autoapi, autoapi_ignore, custom_css, custom_js lookups to get_config_flex for flexible key matching
• Add html_extra_path support in build.py and conf.py.j2, with relative-to- absolute path conversion
• Add tests for get_config_flex and html_extra_path

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.48%. Comparing base (ddc99c5) to head (db97311).
⚠️ Report is 17 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #102      +/-   ##
==========================================
+ Coverage   78.84%   80.48%   +1.63%     
==========================================
  Files           8        8              
  Lines        1130     1230     +100     
  Branches      134      138       +4     
==========================================
+ Hits          891      990      +99     
  Misses        201      201              
- Partials       38       39       +1     

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