Make "About" link point to current website and not open in a new tab

[GuillaumeGomez]

When I was working on #2218, I realized (after some time...) that it was actually redirecting me to docs.rs every time and not simply use my local instance.

So this PR changes two things:

1. Use a relative URL instead of pointing to docs.rs.
2. Since it's the same website, not open this link into a new tab.

[syphar]

I like the change, will approve after testing when I'm back next week