Ignore some generated and developer files

Ignore doxygen-generated documentation.

Ignore ctags/etags/global files and gdb configuration file.
diff --git a/.gitignore b/.gitignore
index fee2a31..1142e03 100644
--- a/.gitignore
+++ b/.gitignore
@@ -23,3 +23,15 @@
 
 # CMake generates *.dir/ folders for in-tree builds (used by MSVC projects), ignore all of those:
 *.dir/
+
+# Generated documentation
+/apidoc
+
+# Common developer files
+/GPATH
+/GSYMS
+/GRTAGS
+/GTAGS
+/TAGS
+/tags
+.gdbinit