Document changes to `cast index`

The `to_type` parameter of `cast index` has been removed. Docs should reflect this change, and the examples should be updated. There's another interesting example for nested mappings in the PR.

See https://github.com/foundry-rs/foundry/pull/2149

Start with the documentation for `cast index` and compare it with PR #2149, especially the nested-mappings example. Update the docs to remove references to the `to_type` parameter and revise the examples; done means the documented command behavior matches the change in that PR.

Written by the indexing model from the issue text.