Reorganize mypy documentation into sections (#5104)

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

Author: Michael0x2a

docs/source/cheat_sheet.rst

@@ -1,7 +1,7 @@
 .. _cheat-sheet-py2:
 
-Mypy syntax cheat sheet (Python 2)
-==================================
+Type hints cheat sheet (Python 2)
+=================================
 
 This document is a quick cheat sheet showing how the `PEP 484 <https://www.python.org/dev/peps/pep-0484/>`_ type
 language represents various common types in Python 2.

docs/source/cheat_sheet_py3.rst

@@ -1,7 +1,7 @@
 .. _cheat-sheet-py3:
 
-Mypy syntax cheat sheet (Python 3)
-==================================
+Type hints cheat sheet (Python 3)
+=================================
 
 This document is a quick cheat sheet showing how the `PEP 484 <https://www.python.org/dev/peps/pep-0484/>`_ type
 language represents various common types in Python 3. Unless otherwise noted, the syntax is valid on all versions of Python 3.

docs/source/index.rst

@@ -10,31 +10,53 @@ Mypy is a static type checker for Python 3 and Python 2.7.
 
 .. toctree::
    :maxdepth: 2
+   :caption: First steps
 
    introduction
    basics
    getting_started
-   builtin_types
+
+.. toctree::
+   :maxdepth: 2
+   :caption: Cheat sheets
+
+   cheat_sheet_py3
+   cheat_sheet
+
+.. toctree::
+   :maxdepth: 2
+   :caption: Type system reference
+
    python2
+   builtin_types
    type_inference_and_annotations
    kinds_of_types
    class_basics
    dynamic_typing
    function_overloading
    casts
    duck_type_compatibility
-   common_issues
    generics
-   supported_python_features
-   additional_features
+
+.. toctree::
+   :maxdepth: 2
+   :caption: Configuring and running mypy
+
    command_line
    config_file
    mypy_daemon
+   supported_python_features
    python36
+   additional_features
+
+
+.. toctree::
+   :maxdepth: 2
+   :caption: Miscellaneous
+
    installed_packages
+   common_issues
    faq
-   cheat_sheet
-   cheat_sheet_py3
    revision_history
 
 Indices and tables