minor bug: top level Makefile

[hjoliver]

The installation instructions say to type "make" in newly unpacked release dirs. This is supposed to create the VERSION file and build the docs. Currently doc build fails and prints a lengthy sphinx build usage message. The VERSION file is still created though, before the doc build fails, so this is a confusing but non-fatal message for new installers.

@sadielbartholomew - this should be a very easy fix, but I'm not entirely sure if you should adapt the Makefile to run "cylc make-docs" or remove doc build from the Makefile (so it only creates the VERSiON file) and expect users to run "cylc make-docs"). (I think there was a small discussion about whether "make" should build docs or not, some time back, but I forget the outcome of that...)

[hjoliver]

(The fix needs to go to 7.8.x too, of course).

[sadielbartholomew]

Sorry for the omission @hjoliver, I did not know (though should have checked) that there was anything that would try to build the docs external to the doc directory.

Currently doc build fails

Just for clarify for any other readers here, this is only the build via the top-level Makefile. cylc make-docs still functions as expected, building cleanly (i.e. no need for major concern!) except for one 'warning' introduced with #2936.

I'll get a PR up (& mirrored to the 7.8.x branch) to address this, along with the warning above, now.

[hjoliver]

Just for clarify for any other readers here,

Correct, thanks for clarifying!