doc building should be more robust

https://github.com/spack/spack/issues/18196

Since 0.1.0 release at least, the doc building seems to be fragile, either for high values of N in -jN, or because configure is not detecting properly that it should not be building something.

I think it's a case of bad use of automake variables, in particular docdir, pkgdocdir, pkghtmldir. We should able to do better.

Bonus points if the patch can be cherry-picked on top of v0.1.x for a bugfix release.