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/book.toml

@@ -9,8 +9,8 @@ site-url = "/docs/"
 [output.html]
 no-section-label = true
 preferred-dark-theme = "light"
-additional-css = ["theme/page-toc.css"]
-additional-js  = ["theme/page-toc.js"]
+additional-css = ["theme/page-toc.css", "theme/plugins.css"]
+additional-js  = ["theme/page-toc.js", "theme/plugins.js"]
 
 [output.html.print]
 enable = false