Refactor build-documentation.sh to use native docc preview command

[Kyle-Ye]

Summary

• Refactored the documentation build script to use docc preview instead of Python HTTP server
• Integrated preview mode directly into the docc command flow for better user experience
• Added support for custom docc HTML rendering directory via DOCC_HTML_DIR environment variable

Changes

Use Native DocC Preview

• Replaced python3 -m http.server with xcrun docc preview command
• Preview mode now uses DocC's built-in preview server with live reloading and better rendering
• Port handling moved earlier in the process for both catalog and non-catalog workflows

Script Improvements

• Consolidated preview logic into main docc command execution (both for .docc catalogs and symbol graphs)
• Removed redundant post-build preview section
• Cleaner control flow with preview mode integrated into the convert/preview decision

Environment Variables

• Added DOCC_HTML_DIR environment variable to specify custom HTML renderer location
• Allows users to point to custom swift-docc-render builds for development

Testing

Tested with:

• ./Scripts/build-documentation.sh --preview - works with native docc preview
• Port conflict handling - correctly detects and handles occupied ports
• Both catalog and non-catalog documentation builds

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 16.42%. Comparing base (54b03b1) to head (e75c0d3).
⚠️ Report is 2 commits behind head on main.

❗ There is a different number of reports uploaded between BASE (54b03b1) and HEAD (e75c0d3). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (54b03b1)	HEAD (e75c0d3)
	2	1

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #554      +/-   ##
==========================================
- Coverage   24.64%   16.42%   -8.23%     
==========================================
  Files         505      481      -24     
  Lines       29204    26838    -2366     
==========================================
- Hits         7198     4408    -2790     
- Misses      22006    22430     +424     

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