docs: Overhaul AI documentation (#29747)

To support the Agentic Editing launch. To dos before merging:

- [ ] Anything marked as `todo!` within `docs/src` (Anyone)
- [x] Check all internal links (Joe)
- Joe: I checked all links and fixed all aside from a few that I
annotated with `todo!` comments
- [ ] Update images (Danilo)
- [ ] Go over / show images of tool cards in agent panel overview
(Danilo)
- [ ] Point billing FAQ to new billing docs (Joe)
- [x] Redirects external links
    - [ ] Needs testing
- [x] Delete old docs
- [ ] Ensure all mentioned bindings use the `{#kb ...}` format and that
they are rendering correctly
- [ ] All agent-related actions are now `agent::` and not `assistant::`
- [x] Mention support of `.rules` files in `rules.md`

Release Notes:

- N/A

---------

Co-authored-by: Joseph T. Lyons <josephtlyons@gmail.com>
Co-authored-by: morgankrey <morgankrey@gmail.com>
Co-authored-by: Smit Barmase <37347831+smitbarmase@users.noreply.github.com>
Co-authored-by: Ben Kunkle <Ben.kunkle@gmail.com>
Co-authored-by: Peter Tripp <peter@zed.dev>
Co-authored-by: Ben Kunkle <ben@zed.dev>
Co-authored-by: Mikayla Maki <mikayla.c.maki@gmail.com>
Co-authored-by: Ben Brandt <benjamin.j.brandt@gmail.com>

docs/README.md

@@ -4,12 +4,14 @@ Welcome to Zed's documentation.
 
 This is built on push to `main` and published automatically to [https://zed.dev/docs](https://zed.dev/docs).
 
-To preview the docs locally you will need to install [mdBook](https://rust-lang.github.io/mdBook/) (`cargo install mdbook`) and then run:
+To preview the docs locally you will need to install [mdBook](https://rust-lang.github.io/mdBook/) (`cargo install mdbook@0.4.40`) and then run:
 
 ```sh
 mdbook serve docs
 ```
 
+It's important to note the version number above. For an unknown reason, as of 2025-04-23, running 0.4.48 will cause odd URL behavior that breaks docs.
+
 Before committing, verify that the docs are formatted in the way prettier expects with:
 
 ```