x/pkgsite: make package path visible after scroll down

[scott-cotton]

What is the URL of the page with the issue?

EG https://pkg.go.dev/github.com/go-air/pal@v0.0.5/memory

What is your user agent?

Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/14.1 Safari/605.1.15

Screenshot

What did you do?

scroll down when there is enough documentation content to scroll.

What did you expect to see?

The path name of the current package at the top of the page (eg GitHub.com/go-air/pal/memory)

What did you see instead?

the name "memory" and a button to copy the path above to the clipboard.

The reason I find this can be confused is that there are lots of packages with the same name and lots of documentation.

One kinda has to scroll up to the package path in order to navigate up.

It would be great if

1. the package path were visible on the top bar after scrolling down.
2. keybindings were in place to scroll to the parent package or module

[julieqiu]

/cc @jinhongy @fflewddur