Document how to build docs for a package

[mortenpi]

Right now, this is what I had to do to get the docs for the latest HDF5.jl package to build locally:

using DocumentationGenerator
packages = DocumentationGenerator.installable_on_version(joinpath(homedir(), ".julia", "registries", "General"), VERSION)
hdf5 = first(filter(p -> p.name == "HDF5", collect(values(packages))))
hdf5 = (;
    hdf5...,
    server_type = "github",
    api_url = "",
    versions = sort(hdf5.versions),
)
DocumentationGenerator.build_documentation([hdf5])