Flesh out and document changeref script

Now that things have settled down, two useful additions could be made to changeref.py:

- redirects in s3_website.yml
- change names in tables of content

These would only be done for doc refs, not section refs

Start by reading changeref.py and s3_website.yml to understand how document references are currently changed. Trace how document references differ from section references, then implement support for redirects and table-of-content change names only for doc refs. Done means both additions work without affecting section refs.

Written by the indexing model from the issue text.