Add complex-related stuff to docs and examples

[Randl]

See #57

[Randl]

I've added a separate simple example with complex entries; I can adda more sophisticated one if you have something in mind

[patrick-kidger]

This LGTM!

Can you add your extra example into mkdocs.yml? Otherwise it won't appear in the generated documentation :)

[patrick-kidger]

Alright, wonderful stuff! This LGTM.

As merging to main will update the docs then I'm going to hold off on merging this PR until we're ready to do the next release, but morally speaking this PR should be considered merged! :)
(I have one other upstream change that I want to land in Equinox first, and then I'm hoping to do a new release of both Equinox and Lineax -- so probably the next week or two.)