I plan to do some more refactoring of docs/Makefile, but first, there's a number of targets that we're not using, and some of which aren't even working. Let's remove them.

We're only really using html, but I left some other html variants in there like dirhtml and singlehtml. linkcheck can also be useful from time to time.

Also removed from docs/make.bat but I don't have Windows to test with, and probably won't many future changes to this file.

If someone now or subsequently says they use any of these, we can add them back.

The non-working targets:

• json
• pickle
• latexpdf (if no latexmk installed)
• info
• doctest