Expand assistant docs (#16501)

This PR significantly expands the assistant documentation, breaking it
out into sections, adding examples and further documenting features.

This PR introduces a convention in docs for swapping keybindings for mac
vs linux:

`<kbd>cmd-enter|ctrl-enter</kbd>`

In the above example, the first will be shown for mac, the second for
linux or windows.

TODO:

- [ ] Fix table style (for `/assistant/configuration`)
- [x] Add script to swap keybindings based on platform
- It should take in this format: [`cmd-n` (mac)|`ctrl-n`(linux)] and
return just the correct binding for the viewer's platform.
- [ ] Add image/video assets (non-blocking)

Release Notes:

- Updated assistant documentation

docs/src/assistant/configuration.md

@@ -0,0 +1,153 @@
+# Configuring the Assistant
+
+## Settings
+
+| key           | type   | default | description                   |
+| ------------- | ------ | ------- | ----------------------------- |
+| version       | string | "2"     | The version of the assistant. |
+| default_model | object | {}      | The default model to use.     |
+
+### Configuring the default model
+
+The `default_model` object can contain the following keys:
+
+```json
+// settings.json
+{
+  "assistant": {
+    "default_model": {
+      "provider": "zed.dev",
+      "model": "claude-3-5-sonnet"
+    }
+  }
+}
+```
+
+## Common Panel Settings
+
+| key            | type    | default | description                                                                           |
+| -------------- | ------- | ------- | ------------------------------------------------------------------------------------- |
+| enabled        | boolean | true    | Disabling this will completely disable the assistant                                  |
+| button         | boolean | true    | Show the assistant icon                                                               |
+| dock           | string  | "right" | The default dock position for the assistant panel. Can be ["left", "right", "bottom"] |
+| default_height | string  | null    | The pixel height of the assistant panel when docked to the bottom                     |
+| default_width  | string  | null    | The pixel width of the assistant panel when docked to the left or right               |
+
+## Example Configuration
+
+```json
+// settings.json
+{
+  "assistant": {
+    "default_model": {
+      "provider": "zed.dev",
+      "model": "claude-3-5-sonnet-20240620"
+    },
+    "version": "2",
+    "button": true,
+    "default_width": 480,
+    "dock": "right",
+    "enabled": true
+  }
+}
+```
+
+## Providers {#providers}
+
+The following providers are supported:
+
+- Zed AI (Configured by default when signed in)
+- [Anthropic](#anthropic)
+- [GitHub Copilot Chat](#github-copilot-chat)
+- [Google Gemini](#google-gemini)
+- [Ollama](#ollama)
+- [OpenAI](#openai)
+- [OpenAI Custom Endpoint](#openai-custom-endpoint)
+
+### Zed AI {#zed-ai}
+
+A hosted service providing convenient and performant support for AI-enabled coding in Zed, powered by Anthropic's Claude 3.5 Sonnet and accessible just by signing in.
+
+### Anthropic {#anthropic}
+
+You can use Claude 3.5 Sonnet via [Zed AI](#zed-ai) for free. To use other Anthropic models you will need to configure it by providing your own API key.
+
+You can obtain an API key [here](https://console.anthropic.com/settings/keys).
+
+Even if you pay for Claude Pro, you will still have to [pay for additional credits](https://console.anthropic.com/settings/plans) to use it via the API.
+
+### GitHub Copilot Chat {#github-copilot-chat}
+
+You can use GitHub Copilot chat with the Zed assistant by choosing it via the model dropdown in the assistant panel.
+
+### Google Gemini {#google-gemini}
+
+You can use Gemini 1.5 Pro/Flash with the Zed assistant by choosing it via the model dropdown in the assistant panel.
+
+You can obtain an API key [here](https://aistudio.google.com/app/apikey).
+
+### Ollama {#ollama}
+
+Download and install Ollama from [ollama.com/download](https://ollama.com/download) (Linux or macOS) and ensure it's running with `ollama --version`.
+
+You can use Ollama with the Zed assistant by making Ollama appear as an OpenAPI endpoint.
+
+1. Download, for example, the `mistral` model with Ollama:
+
+   ```sh
+   ollama pull mistral
+   ```
+
+2. Make sure that the Ollama server is running. You can start it either via running the Ollama app, or launching:
+
+   ```sh
+   ollama serve
+   ```
+
+3. In the assistant panel, select one of the Ollama models using the model dropdown.
+4. (Optional) If you want to change the default URL that is used to access the Ollama server, you can do so by adding the following settings:
+
+```json
+{
+  "language_models": {
+    "ollama": {
+      "api_url": "http://localhost:11434"
+    }
+  }
+}
+```
+
+### OpenAI {#openai}
+
+<!--
+TBD: OpenAI Setup flow: Review/Correct/Simplify
+-->
+
+1. Create an [OpenAI API key](https://platform.openai.com/account/api-keys)
+2. Make sure that your OpenAI account has credits
+3. Open the assistant panel, using either the `assistant: toggle focus` or the `workspace: toggle right dock` action in the command palette (`cmd-shift-p`).
+4. Make sure the assistant panel is focused:
+
+   ![The focused assistant panel](https://zed.dev/img/assistant/assistant-focused.png)
+
+The OpenAI API key will be saved in your keychain.
+
+Zed will also use the `OPENAI_API_KEY` environment variable if it's defined.
+
+#### OpenAI Custom Endpoint {#openai-custom-endpoint}
+
+You can use a custom API endpoint for OpenAI, as long as it's compatible with the OpenAI API structure.
+
+To do so, add the following to your Zed `settings.json`:
+
+```json
+{
+  "language_models": {
+    "openai": {
+      "api_url": "http://localhost:11434/v1"
+    }
+  }
+}
+```
+
+The custom URL here is `http://localhost:11434/v1`.