Docs: remove unused Makefile targets

[hugovk]

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

[radarhere]

Here's my investigation of the non-working targets,

• ‘json’ and ‘pickle’ raise an error

sphinx.errors.ExtensionError: Handler <function html_page_context at 0x107d27920> for event 'html-page-context' threw an exception (exception: 'pagename')

• ‘latexpdf’ does indeed raise

latexmk: No such file or directory

• ‘info’ doesn’t raise an error. It does produce a number of warnings

WARNING: a suitable image for texinfo builder not found
warning: @footnote should not appear on @deffn line
warning: @subsubsection missing argument

• ‘doctest’ raises an error

sphinx.errors.SphinxError: Builder name doctest not registered or available through entry point