Do not expose `pwnlib.gdb.Gdb` / `.Breakpoint` to documentation on main GDB page

This `pwnlib.gdb.Gdb` object / module should not be exposed, or should be exposed under a submodule, as it's only used for remote Python API stuff.

https://docs.pwntools.com/en/latest/gdb.html#pwnlib.gdb.Gdb

It looks like we can just add an `exclude-members` directive to the `automobile::` statement in `gdb.rst` to do this.