Add configurable keyboard shortcut to focus search input (#1411)

Adds a keyboard shortcut to use Ctrl/Cmd and a configurable key to focus the search input. The key is configurable with `search.focus_shortcut_key`; enables it on the core docs site with `'k'` (default on many sites).

---------
Co-authored-by: Matt Wang <mxw@cs.washington.edu>

docs/configuration.md

@@ -66,6 +66,8 @@ search:
   # Enable or disable the search button that appears in the bottom right corner of every page
   # Supports true or false (default)
   button: false
+  # Focus the search input by pressing `ctrl + focus_shortcut_key` (or `cmd + focus_shortcut_key` on macOS)
+  focus_shortcut_key: 'k'
 ```
 
 ## Mermaid Diagrams

docs/search.md

@@ -94,6 +94,21 @@ The search button displays in the bottom right corner of the screen and triggers
 search.button: true
 ```
 
+### Focus search bar with a keyboard shortcut
+
+Just the Docs supports focusing the search bar input with a keyboard shortcut. After setting the `search.focus_shortcut_key` config item key, users who press <kbd>Ctrl</kbd> + `search.focus_shortcut_key` (or on macOS, <kbd>Command</kbd> + `search.focus_shortcut_key`) will focus the search bar.
+
+Note that this feature is **disabled by default**. `search.focus_shortcut_key` should be a [valid value from `KeyboardEvent.key`](https://developer.mozilla.org/en-US/docs/Web/API/KeyboardEvent/key); this involves all ASCII alphanumeric values, as well as modifier keys.
+
+For example,
+
+```yaml
+search:
+    focus_shortcut_key: 'k'
+```
+
+Will make <kbd>Ctrl</kbd> + <kbd>K</kbd> focus the search bar for Windows users (and <kbd>Command</kbd> + <kbd>K</kbd> on macOS).
+
 ## Hiding pages from search
 
 Sometimes you might have a page that you don't want to be indexed for the search nor to show up in search results, e.g., a 404 page.