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/theme/index.hbs

@@ -23,6 +23,7 @@
         <link rel="stylesheet" href="{{ path_to_root }}css/variables.css">
         <link rel="stylesheet" href="{{ path_to_root }}css/general.css">
         <link rel="stylesheet" href="{{ path_to_root }}css/chrome.css">
+        <link rel="stylesheet" href="{{ path_to_root }}plugins.css">
         {{#if print_enable}}
         <link rel="stylesheet" href="{{ path_to_root }}css/print.css" media="print">
         {{/if}}
@@ -251,9 +252,10 @@
         <script src="{{ path_to_root }}highlight.js"></script>
         <script src="{{ path_to_root }}book.js"></script>
 
-        <!-- Custom JS scripts -->
+         <!-- Custom JS scripts -->
         {{#each additional_js}}
         <script src="{{ ../path_to_root }}{{this}}"></script>
+        <script src="{{ path_to_root }}plugins.js"></script>
         {{/each}}
 
         {{#if is_print}}