Add `doctest-%` pattern target

> Instead, it'd be good to have a generic target e.g.
>
> ```
> doctest-%:
> cd $* && $(DOCTEST)
> ```
>
> -- [doctest-% suggestion](https://matrix.to/#/!YWYQDCuIpuQHzGrLzd:matrix.org/$X8_Imm8BwEiTaNyqLnGGmXXRsiyZqVDbKkxLfg4fEKk?via=matrix.org&via=kf8nh.com&via=tchncs.de)

Start by locating the build configuration that defines DOCTEST and existing doctest targets. Add the generic doctest-% target described in the issue, then verify that invoking it with a directory runs the doctests there.

Written by the indexing model from the issue text.