Crossreferences to standard library in mypy docs, part 7

[hoefling]

Added references to the protocols.rst. This is the last part of splitting up the changes in #7624 into more readable PRs.

[hoefling]

Please advise whether the refs in the section titles are useful.

[gvanrossum]

I prefer not to have them (consistent with the earlier policy against code in titles. But then we need to edit the body text to include a ref, since it's still useful to have it.

[hoefling]

Maybe a generic line like "See also ProtocolName documentation." to each section? The stdlib docs don't shed more light on protocol types TBH; PEP 544 is the information source if one needs details (and it's not even mentioned in the typing documentation!).

Or maybe a line like "The ProtocolName type is defined in typing module.", just to point out where the type can be imported from?

[gvanrossum]

Let's put a line like

See also :py:class:`Iterable`.

at the end of each section.

[ilevkivskyi]

(and it's not even mentioned in the typing documentation!)

It is in the relevant version (i.e. Python 3.8): https://docs.python.org/3.8/library/typing.html

[hoefling]

I have removed the remote branch by accident, didn't mean to close this PR.