This PR adds a mdbook preprocessor for supporting Zed's docs.
This initial version adds the following custom commands:
**Keybinding**
`{#kb prefix::action_name}` (e.g. `{#kb zed::OpenSettings}`)
Outputs a keybinding template like `<kbd
class="keybinding">{macos_keybinding}|{linux_keybinding}</kbd>`. This
template is processed on the client side through `mdbook` to show the
correct keybinding for the user's platform.
**Action**
`{#action prefix::action_name}` (e.g. `{#action zed::OpenSettings}`)
For now, simply outputs the action name in a readable manner. (e.g.
zed::OpenSettings -> zed: open settings)
In the future we'll add additional modes for this template, like create
a standard way to render `{action} ({keybinding})`.
## Example Usage
```
To open the assistant panel, toggle the right dock by using the {#action workspace::ToggleRightDock} action in the command palette or by using the
{#kb workspace::ToggleRightDock} shortcut.
```
Release Notes:
- N/A
