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/billing.md

@@ -0,0 +1,27 @@
+# Billing
+
+We use Stripe as our billing and payments provider. All Pro plans require payment via credit card.
+For invoice-based billing, a Business plan is required. Contact sales@zed.dev for more information.
+
+## Settings {#settings}
+
+You can access billing settings at [zed.dev/account](https://zed.dev/account).
+Clicking the button under Account Settings will navigate you to Stripe’s secure portal, where you can update all billing-related settings and configuration.
+
+## Billing Cycles {#billing-cycles}
+
+Zed is billed on a monthly basis based on the date you initially subscribe.
+
+We’ll also bill in-month for additional prompts used beyond your plan’s prompt limit, if usage exceeds $20 before month end. See [usage-based pricing](./plans-and-usage.md#ubp) for more.
+
+## Invoice History {#invoice-history}
+
+You can access your invoice history by navigating to [zed.dev/account](https://zed.dev/account) and clicking [button].
+
+From Stripe’s secure portal, you can download all current and historical invoices.
+
+## Updating Billing Information {#updating-billing-info}
+
+You can update your payment method, company name, address, and tax information through the billing portal.
+
+Please note that changes to billing information will **only** affect future invoices — **we cannot modify historical invoices**.