Lots of small fixes to docs in the turbo-tasks crate (#8651)
### Description
Docs live here:
https://turbopack-rust-docs.vercel.sh/rustdoc/turbo_tasks/index.html
- Fixed lots of links.
- Export `VcCast`, `VcValueTypeCast`, and `VcValueTraitCast`, as they're
used in some type signatures, and some documentation is impossible to
follow without it. This introduces the sealed trait pattern to `VcCast`
to continue to strongly prevent foreign implementations.
### Testing Instructions
```
cargo doc --keep-going -p turbo-tasks
cargo doc --keep-going --document-private-items -p turbo-tasks
```
