Swap to Zensical

[davep]

Experiment with swapping from MkDocs to Zensical.

Currently keeping this in draft because I'm finding that Zensical takes about 10 times longer to generate the docs than MkDocs does; it seems to get bogged down badly when doing the screenshots.

To compare, here's the times when building the docs on my 2019 Intel MacBook Pro:

Executed in   57.17 secs    fish           external
   usr time   38.55 secs    0.22 millis   38.55 secs
   sys time    1.30 secs    1.56 millis    1.30 secs

same machine, but with this PR's branch (and so using Zensical):

Executed in  541.86 secs    fish           external
   usr time  542.42 secs    0.26 millis  542.42 secs
   sys time    3.39 secs    1.82 millis    3.39 secs

In both cases this was building the docs from scratch, clean clone, make setup performed first.

[davep]

See https://fosstodon.org/@pawamoy/116206445881190504