Expand "getting started" docs to discuss type hints in more detail (#6226)

This pull request expands our "getting started" docs so they discuss how
to use type hints in more detail.

In general, I'm hoping this pull request will accomplish two main
things:

1. Give users a better "tour" of the available PEP 484 types.

2. Do a better job of giving users a "heads-up" of common gotchas:
   for example, how dynamically typed functions aren't type-checked.

Here's a list of specific changes I made:

1. I combined "Installing mypy" and "Running mypy" into one section.

2. I modified the "Function signatures" section to focus more on the
   distinction between dynamic and static typing: I suspect that's
   a common tripup new users run into.

   I also changed the examples so they'll actually produce a runtime
   error if you misuse them.

3. I moved the existing text on annotating no-argument functions or
   functions with default arguments into a separate "More function
   signatures" to try and keep the new "Function signatures and and
   dynamic vs static typing" section focused.

4. I expanded the section about using the "typing" module to be more
   thorough. In particular, I wanted to expose the user to the concept
   of concrete generic types (List), abstract generic types/protocols
   (Iterable), and to more non-conventional types like Union.

   I think these three examples are broadly representative of the
   different "flavors" of PEP 484 types and so should help new users
   be a little more aware of the kinds of things they can do with types.

   The main thing that's missing is an introduction to custom generic
   classes and functions: that's probably too complex of a topic to
   belong in this page.

5. I added a section on local type inference: it's a new concept for
   people who are more used to languages like Java or C# so felt like
   it was worth mentioning.

   This section also covers the final stumbling block I think new users
   are likely to run into: the "Need type annotation for 'variable'"
   error message.

6. I added a section on customizing mypy via the command line flags.
   I don't think enough users are aware that you can do this, despite
   how useful the flags can be.

Author: Michael0x2a

docs/source/command_line.rst

@@ -336,7 +336,7 @@ potentially problematic or redundant in some way.
     a ``# type: ignore`` comment on a line that is not actually
     generating an error message.
 
-    This flag, along with the ``--warn-unsued-casts`` flag, are both
+    This flag, along with the ``--warn-unused-casts`` flag, are both
     particularly useful when you are upgrading mypy. Previously,
     you may have needed to add casts or ``# type: ignore`` annotations
     to work around bugs in mypy or missing stubs for 3rd party libraries.