Slow documentation builds

[dkfellows]

Is it just me or is the time to build the documentation with Sphinx very slow? Previous projects where I've used it have tended to be able to write tens of files per second, whereas hpcflow seems to slow to the point of taking a second or more for each output file (looking at things when they're building on Github). That's weird!

Dodgy hunch-based analysis

I'm not at all sure what's causing this (or why matflow isn't suffering) but I suspect it might be due to some sort of quadratic complexity behaviour triggered by the list of all versions of the documentation that gets embedded in each page. By now, hpcflow has a very large number of alpha versions...

[SJaffa]

Just found a few other people suffering from slow builds that might contain useful info:

sphinx-doc/sphinx#4576
https://stackoverflow.com/questions/51986357/how-can-i-make-sphinx-build-build-faster

[dkfellows]

I've never delved deep into the guts of Sphinx, and the parts that I have seen have not encouraged me to change that.