docs: Consolidate coverage documentation into single guide

Consolidate 20 separate coverage markdown files into one comprehensive
COVERAGE_GUIDE.md that includes:

- Quick start instructions
- Current status and metrics
- Key discoveries (importance of -a flag)
- Complete usage guide (local and CI)
- Test suite expansion details
- Troubleshooting section
- Advanced topics and future work
- FAQs and quick reference

Also update .gitignore to exclude:
- coverage_results/ directories
- .gcov, .gcda, .gcno files
- coverage HTML/XML/text reports

This reduces documentation clutter while preserving all important
information in a single, searchable guide.

Files consolidated:
- README_COVERAGE.md
- COVERAGE_QUICK_REFERENCE.md
- COVERAGE_FINAL_SUMMARY.md
- POSTPROCESS_COVERAGE_RESULTS.md
- NEXT_STEPS.md
- And 15 other coverage documentation files

Kept separate:
- TEST_SUITE_EXPANSION_IMPLEMENTED.md (technical implementation details)
- CI_COVERAGE_IMPLEMENTATION_COMPLETE.md (project status summary)

Author: sbryngelson

.gitignore

@@ -8,6 +8,17 @@ yarn.lock
 src/*/include/case.fpp
 src/*/autogen/
 
+# Coverage results
+coverage_results/
+coverage_results_*/
+*.gcov
+*.gcda
+*.gcno
+coverage.html
+coverage.xml
+coverage_summary.txt
+coverage_report.html
+
 *.swo
 *.swp