x/pkgsite: README images should be proxied to protect user privacy

The overview page links directly to images referenced from the readme markdown. These references allow third parties to track users on pkg.go.dev.

Github proxies images referenced from the readme to prevent such tracking. See the following for more information:

• https://github.blog/2014-01-28-proxying-user-images/
• https://help.github.com/en/github/authenticating-to-github/about-anonymized-image-urls

pkg.go.dev should do the same.

What is the URL of the page with the issue?

https://pkg.go.dev/github.com/labstack/echo?tab=overview and many other ?tab=overview pages.

What did you do?

I visited the page.

What did you expect to see?

I expected to see requests to pkg.go.dev and it's trusted partners.

What did you see instead?

I saw image requests to sourcegraph.com, img.sheilds.go and goreportcard.com.

[dmitshur]

/cc @julieqiu per owners.