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/inline-assistant.md

@@ -0,0 +1,35 @@
+# Inline Assistant
+
+## Using the Inline Assistant
+
+You can use `ctrl-enter` to open the Inline Assistant nearly anywhere you can enter text: editors, the agent panel, the prompt library, channel notes, and even within the terminal panel.
+
+The Inline Assistant allows you to send the current selection (or the current line) to a language model and modify the selection with the language model's response.
+
+You can use `ctrl-enter` to open the inline assistant nearly anywhere you can write text: editors, the Agent Panel, the Rules Library, channel notes, and even within the terminal panel.
+
+You can also perform multiple generation requests in parallel by pressing `ctrl-enter` with multiple cursors, or by pressing `ctrl-enter` with a selection that spans multiple excerpts in a multibuffer.
+
+## Context
+
+You can give the Inline Assistant context the same way you can in the agent panel, allowing you to provide additional instructions or rules for code transformations with @-mentions.
+
+A useful pattern here is to create a thread in the [Agent Panel](./agent-panel.md), and then use the `@thread` command in the Inline Assistant to include the thread as context for the Inline Assistant transformation.
+
+## Prefilling Prompts
+
+To create a custom keybinding that prefills a prompt, you can add the following format in your keymap:
+
+```json
+[
+  {
+    "context": "Editor && mode == full",
+    "bindings": {
+      "ctrl-shift-enter": [
+        "assistant::InlineAssist",
+        { "prompt": "Build a snake game" }
+      ]
+    }
+  }
+]
+```