Reduce usage of automodule in Layout docs section
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.

jonathanslenders/python-prompt-toolkit