Rebuild doxygen docs when headers change

[roberth]

Motivation

Make it easy to review rendered docs.

Context

Priorities and Process

Add 👍 to pull requests you find important.

The Nix maintainer team uses a GitHub project board to schedule and track reviews.

[edolstra]

I'm a bit worried that in conjunction with #10527 this will degrade the edit-compile experience, since every change to a header file will now trigger a doxygen rebuild, which is pretty noisy.

[roberth]

I'd checked that make default check && make install && make installcheck doesn't run it. That's what we recommend on the hacking page, but installPhase will run it, you're right.

Is a fast manual make sufficient, or should I add an override to installTargets? Can be done in the devshell, so that it doesn't pollute package.nix.
It's not actually that slow btw; just very noisy.