Improve .gitignore grouping and documentation

.gitignore

@@ -1,8 +1,17 @@
+# CMake build artifacts:
 CMakeCache.txt
 CMakeFiles
 CTestTestfile.cmake
 cmake_install.cmake
 Testing
+# CMake generates *.dir/ folders for in-tree builds (used by MSVC projects), ignore all of those:
+*.dir/
+# MSVC files generated by CMake:
+/*.sln
+/*.vcxproj
+/*.filters
+
+# Test coverage build artifacts:
 Coverage
 *.gcno
 *.gcda
@@ -10,11 +19,6 @@ Coverage
 # generated by scripts/memory.sh
 massif-*
 
-# MSVC files generated by CMake:
-/*.sln
-/*.vcxproj
-/*.filters
-
 # MSVC build artifacts:
 *.exe
 *.pdb
@@ -24,8 +28,6 @@ massif-*
 # Python build artifacts:
 *.pyc
 
-# CMake generates *.dir/ folders for in-tree builds (used by MSVC projects), ignore all of those:
-*.dir/
 
 # Editor navigation files:
 /GPATH