Add a prominent link to the standard library docs

[odenix]

Motivation: The standard library docs are some of the most important docs but currently very hard to find. I've noticed that even fairly advanced users have never heard of them and complain about "zero docs".

[bioball]

We actually used to have this as a link, but took it out because it pointed to the same website as Package Docs. But, I agree, it made the standard library too hard to find as a result.