DM-40495: Better error reporting from Sphinx linkcheck

[rra]

Sphinx linkcheck doesn't produce a summary of failed links, but it does write one to output.txt. Move the linkcheck command into the Makefile so that we can use shell trickery to display the contents of that file on failure.

Add a help target as the default target in Makefile.

[rra]

I know the long-term intention is to add a documenteer command, but I think this shouldn't hurt in the short term until you get a chance to work on that.

[jonathansick]

This is great to have for now. I wonder if tox running make means that make is running Python inside tox's environment? I think so?

[rra]

Good question! I tested that by running tox run -e docs-linkcheck outside of a virtualenv, and it succeeded despite the fact that a bunch of the dependencies for building the documentation are not installed system-wide, so I am fairly sure that's the case. (And that would make sense since I think the virtualenv is controlled by environment variables that should be inherited.)