These docs are available from two different paths:

```
/std/string/struct.String.html
/std/primitive.str.html
```

Because they're at different levels, we can't construct a single relative path that works for both. However, we can ask the tooling to generate the correct relative path for each source with an `intra-doc-link`.

See fb26b0c. This commit also removes the exclusion for the String docs page, but there's still a few broken links.

At least locally, this required the fix in librustdoc: else, I was seeing broken links pointing into the source of the module being documented rather than linking to the `std/primitive.char.html#method.FOO` page as was likely intended.