An attempt at making the docs shorter and sweeter including various small cleanup items.
- Reorder the TOC on the sidebar for the user guides to be more linear based on a user's journey.
- Put the batching content under the performance guide.
- Remove the AIR guide (AIR users already have a serving guide).
- Combine the `ServeHandle` and model composition pages into a single guide. We may want to revisit this in the future but for now better to have it in a single place instead of duplicated (with links going to both).
- Fix the index page for the user guides to match the TOC sidebar.
- Rename a few pages for clarity & consistency.
- Remove some now-redundant content (old ML models user guide).
