Allow linking to symbols inside source documentation

[yorickpeterse]

Description

Documentation of one symbol may refer to another symbol, i.e like so:

`Int.until`

We should turn such references into actual links to the fully qualified symbol URL. As a starting point, this should probably only support fully qualified references, e.g. std.int.Int.until, such that we don't need to maintain a giant hash map of all possible symbols for every page. I'm not entirely sure yet how to best implement this.

Related work

No response