gitignore: Pin directory exclusions to top-level

Previously, new files in test directories named `Build*` on
case-insensitive filesystems were being incorrectly ignored.

While at it, add more comments justifying some entries.

.gitignore

@@ -1,27 +1,30 @@
+/CMakeUserPresets.json
+
 # Common build directories
-build*/
+/build*/
 
-# Exclude MacOS Finder files.
+# MacOS Finder files.
 .DS_Store
 
-*.user*
-
+# Python compile output.
 *.pyc
 
-Help/_generated
-Testing
-CMakeUserPresets.json
+# See Utilities/Sphinx/tutorial_archive.cmake
+/Help/_generated
 
-# Visual Studio work directory
-.vs/
-# Visual Studio build directory
-out/
+# CLion work directory
+/.idea/
+# CLion build directories
+/cmake-build-*/
+
+# QtCreator files.
+/CMakeLists.txt.user*
 
 # Visual Studio Code
-.vscode/
-.cache/
+/.vscode/
+/.cache/
 
-# CLion work directory
-.idea/
-# CLion build directories
-cmake-build-*/
+# Visual Studio work directory
+/.vs/
+# Visual Studio build directory
+/out/