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/plans-and-usage.md

@@ -0,0 +1,39 @@
+# Plans and Usage
+
+To view your current usage, you can visit your account at [zed.dev/account](https://zed.dev/account).
+You’ll also find usage meters in-product when you’re nearing the limit for your plan or trial.
+
+## Available Plans {#plans}
+
+For costs and more information on pricing, visit [Zed’s pricing page](https://zed.dev/pricing).
+
+Please note that if you’re interested in just using Zed as the world’s fastest editor, with no AI or subscription features, you can always do so for free, without [authentication](../accounts.md).
+
+## Usage {#usage}
+
+- A `prompt` in Zed is an input from the user, initiated on pressing enter, composed of one or many `requests`. A `prompt` can be initiated from the Agent Panel, or via Inline Assist.
+- A `request` in Zed is a response to a `prompt`, plus any tool calls that are initiated as part of that response. There may be one `request` per `prompt`, or many.
+
+Most models offered by Zed are metered per-prompt.
+Some models that use large context windows and unlimited tool calls ([“Max Mode”](./models.md#max-mode)) count each individual request within a prompt against your prompt limit, since the agentic work spawned by the prompt is expensive to support.
+
+See [the Models page](./models.md) for a list of which subset of models are metered by request.
+
+Plans come with a set amount of prompts included, with the number varying depending on the plan you’ve selected.
+
+## Usage-Based Pricing {#ubp}
+
+You may opt in to usage-based pricing for prompts that exceed what is included in your paid plan from [your account page](https://zed.dev/account).
+
+Usage-based pricing is only available with a paid plan, and is exclusively opt-in.
+From the dashboard, you can toggle usage-based pricing for usage exceeding your paid plan.
+You can also configure a spend limit in USD.
+Once the spend limit is hit, we’ll stop any further usage until your prompt limit resets.
+
+We will bill for additional prompts when you’ve made prompts totaling $20, or when your billing date occurs, whichever comes first.
+
+Cost per request for each model can be found on [the models page](./models.md).
+
+## Business Usage {#business-usage}
+
+Email [sales@zed.dev](mailto:sales@zed.dev) with any questions on business plans, metering, and usage-based pricing.