Remove mention of submodules from the documentation

At least the obvious ones.

We have a peculiar structure here, most of the stuff is hidden in submodules but they don't matter because the public-facing API is imported into the main module through __init__.py. Sphinx still shows the submodules as if they matter though.

This commit drops the bits from sphinx to hide the fact we're mostly in submodules because it really doesn't matter to the user.

cc @bentiss, any comments? just looking for a second opinion here.