x/pkgsite: add option to refresh a package on the package page

[julieqiu]

Following up on #41055 (comment):

At the moment, we provide support for refreshing a package by visiting pkg.go.dev/<path>@master, but it's not obvious that this feature is available.

We should add a way to make this process more obvious, such as by adding an "Update this package" button.

/cc @jamalc @jba

At the moment, we provide support for refreshing a package by visiting pkg.go.dev/<path>@master

That doesn't seem to work: https://pkg.go.dev/gitlab.com/opennota/morph@master

The latest version is tagged v1.0.2, but https://pkg.go.dev/gitlab.com/opennota/morph still shows v1.0.1.

[jba]

It's now at v1.0.2. Perhaps the usual processing lag.