Add gcovr UI customization templates and build scripts

[julioest]

Summary

• Add customizable HTML templates for gcovr coverage reports with sidebar navigation
• Add supporting scripts for local development on macOS
• Update build process to work across both local/macOS and CI/Linux environments

Changes

Templates (templates/html/)
Custom Jinja2 templates that override gcovr's default HTML output:

• Sidebar navigation with collapsible directory tree
• Improved styling and layout
• Source file viewer with line-by-line coverage highlighting

Scripts

• build_tree.py - Generates tree.json for sidebar directory navigation
• gcovr_wrapper.py - Wrapper for gcovr to handle Cobertura XML input

Build Script

• Auto-detects paths from .info files for portability across machines
• macOS workaround: converts lcov .info files to Cobertura XML since gcovr cannot read .gcda files directly on macOS
• CI/Linux path uses gcovr directly with native coverage data

Test Plan

• Run ./build.sh on macOS with a coverage_filtered.info file
• Verify HTML report generates with working sidebar navigation
• Run on CI/Linux and verify report generation

[sdarwin]

What is the output of scripts/build_tree.py? One file or a series of files? What are the filenames, where does the output go. Could such info be mentioned in a comment at the top of build_tree.py, or elsewhere in a doc.

This repo is slightly temporary, although it's good for development purposes. Ultimately the templates will be located in https://github.com/cppalliance/ci-automation for deployment. You have marked the PR as a draft, but since the whole repo is like a develop branch, we can merge when you believe it's ready. At that time, remove 'draft' status. let me know.

[julioest]

What is the output of scripts/build_tree.py?

So, It creates a tree.json file. To build out the tree on the frontend.

Also, I pushed the files that this outputs, so you could see the UI. Open the json/gcovr/index.html file

[sdarwin]

The new pages look great. I will deploy it to production.

[sdarwin]

Results may be seen here:
boostorg/url#969
boostorg/json#1138