Reorganize extension language auto-loading docs.

	* gdb.texinfo (Auto-loading): Move menu up.  Move discussion of
	auto-loaded objfile scripts and .debug_gdb_scripts section to their
	corresponding section in Extending GDB.
	(Extending GDB): Move menu up.  New menu item "Auto-loading
	extensions".
	(Sequences): New menu item "Auto-loading sequences".
	(Auto-loading sequences): New node.
	(Python): Rename section from Scripting GDB to Extending GDB.
	(Python Auto-loading): Update xref, refer to "Auto-loading extensions".
	Move docs on ways to auto-load extensions to ...
	(Auto-loading extensions): ... here.  New node.