Warn if the package module is missing a docstring

[DilumAluthge]

Copying from https://github.com/JuliaRegistries/RegistryHealthChecks.jl/issues/2

---

If a module is missing a docstring and a user tries help?> Package, currently nothing shows. JuliaLang/julia#39093 aims to print the README contents

But, it would be good to promote the addition of carefully crafted docstrings for the main module in the package, as they would likely be better crafted for purpose.

Could the merge bot warn if there isn't one?
Perhaps there are other best-practices to check for too?

[DilumAluthge]

cc: @IanButterworth

[giordano]

One problem is that at the moment we don't parse nor run the code of the package. In particular, we don't want to install it, nor pull all of its dependencies, Pkg isn't involved at all here.