Reorganize mypy documentation into sections

[Michael0x2a]

This pull request reorganizes the documentation into different sections. This change should hopefully:

1. Make it a little easier for new users to navigate the docs.
2. Make the nav sidebar feel a little less overwhelming.
3. Make it more obvious which parts of the documentation are about type
   hint semantics vs which are about specifically mypy.

This pull request also retitled the cheat sheets: the phrase "mypy syntax" didn't feel quite accurate to me.

[Michael0x2a]

Also, I uploaded a (temporary) build of this branch to Read the Docs if anybody wants to quickly see what it looks like. The only real changes are to the index page and the nav.

• Current: http://mypy.readthedocs.io/en/latest/index.html
• PR: https://michael0x2a-mypy.readthedocs.io/en/add-sections-to-docs/

[msullivan]

This is great! The sections make it much less wall of text.

[gvanrossum]

Thanks! This reminds me -- we need a section on Python 3.7 features.