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/src/ai/subscription.md

@@ -0,0 +1,13 @@
+# Subscription
+
+Zed's hosted models are offered via subscription to Zed Pro or Zed Business.
+
+> Using your own API keys is **_free_** - you do not need to subscribe to a Zed plan to use our AI features with your own keys.
+
+See the following pages for specific aspects of our subscription offering:
+
+- [Plans and Usage](./plans-and-usage.md): Outlines Zed's plans, how usage is measured, and usage-based pricing for overages.
+
+- [Billing](./billing.md): Billing policies and procedures, and how to update or view various billing settings.
+
+- [Models](./models.md): Overview of the models offered by Zed's subscriptions.