Add make [-C docs] htmllive to rebuild and reload HTML files

[hugovk]

The existing make livehtml uses https://github.com/lepture/python-livereload and builds the docs and opens them in the browser, but if I edit an RST file, it doesn't rebuild the file and reload the site.

Let's replace it with make htmllive which uses https://github.com/sphinx-doc/sphinx-autobuild, and does the same as above, but also rebuilds changed RST files and reloads the browser.

This is what is used in CPython docs (and devguide and PEPs) and it's great to use. So I used the same htmllive target name because I'm used to typing it. I guess no-one has been using the old livehtml target in a while, as we've not noticed it working.

I added sphinx-autobuild to the list of optional docs dependencies. We could only install this when doing a livehtml build, but it's easier to always install it. Happy to change if preferred.

Also deleted docs/Guardfile, I don't think it's needed?

[radarhere]

Suggested change

	@echo " htmlview to open the index page built by the html target in your browser"
	@echo " htmllive to rebuild and reload HTML files in your browser"
	@echo " htmllive to rebuild and reload HTML files in your browser"
	@echo " htmlview to open the index page built by the html target in your browser"

If you, and you may not, make the other suggested change to be alphabetical, maybe this group of html* targets should be in the same order?

[hugovk]

Let's keep this as is -- it's going from simpler action to more complex.

And actually, I'll revert the change to the top-level Makefile too.