Reduce usage of automodule in Layout docs section

Using automodule multiple times adds the same module docstring multiple
times, bugging out the TOC tree and links. Module docstring for
prompt_toolkit.layout should also be put in the leading section, not
under "The layout class itself".

This commit fixes the above by replacing the usage of `.. automodule::`
with `.. autoclass::`. The `..automodule::` clause is used only in the
leading section, where the module introduction text should land.