x/pkgsite: show README.md of "subpackages" in overview tab

[sbinet]

currently, go.dev does a nice job for presenting the README.md page for the "top-level" module (usually corresponding to the root of a repository) when clicking on the "overview" page:

• https://pkg.go.dev/go-hep.org/x/hep?tab=overview

it would be nice if, when going to "subdirectories", the "overview" tab could display the corresponding README.md page, if any.

i.e., it would be nice if the following link ("hplot" subdir of go-hep.org/x/hep module):

• https://pkg.go.dev/go-hep.org/x/hep@v0.26.0/hplot?tab=overview

would display the README page of:

• https://github.com/go-hep/hep/blob/master/hplot/README.md
  (with all the nice images)

that would help with creating nice looking documentation+examples for packages that need to show images (which isn't the core thing of godoc per se)

[julieqiu]

Thanks for the feedback! We are planning to add this feature in an upcoming round of design changes.