Update Devguide about building documentation.

[JDLH]

The Devguide section 7.5.1 "Building the documentation" / "Using make / make.bat" is out of date. The document lists 10 documentation targets for make. The Doc/Makefile lists 17.

One important omission is make check, which looks for errors in reStructuredText syntax, using rstlint.py. It's important for contributors to know about and to run make check, because with the move to GitHub, the project is running make check after pulling in new code. If contributors haven't cleaned up their submissions, the build fails.

I suggest that the following improvements be made:

1. rewrite the bullet list "Available make targets are:" in 7.5.1 https://cpython-devguide.readthedocs.io/documenting.html#using-make-make-bat to match the list of targets in Doc/Makefile:21-38.
2. add a comment that make help will give the most up-to-date list of targets from the makefile, which might be better than the list in the Devguide.
3. add make help to both Doc/Makefile:21-38 and Devguide 7.5.1.
4. add to Devguide 6.3. "Helping with the Developer’s Guide" an instruction to run make check before submitting a pull request to the Devguide.

(This issue is moved over from http://bugs.python.org/issue29563 .)

[JDLH]

Carrying over comment from http://bugs.python.org/issue29563, by Zachary Ware (zach.ware), 2017-02-15 03:58:
"I'd suggest replacing as much as possible in the devguide with a link to Doc/README.rst (or Doc/README.txt, if that's what it still is). Other improvements should be made in the README."