Clean up .gitignore

Add comments for documenting the entries.

Remove IDE hidden folders, they should be ignored globally on the
system, not per-project.

.gitignore

@@ -1,15 +1,19 @@
-target/
+# Common build directory names
-build/
 _build/
+build/
 builddir/
-build-aux/app
+
+# Cargo
+target/
+
+# Flatpak
 .flatpak-builder
-src/config.rs
-*.ui.in~
-*.ui~
 .flatpak
-subprojects/libadwaita
-subprojects/gtksourceview
-.vscode
 .fenv
-.zed
+
+# Temporary files
+*.ui.in~
+*.ui~
+
+# Dynamically-generated file
+src/config.rs