[doc] Online documentation export issue

### Description

The CSS for https://opentitan.org/book does not seem to be defined for printable media:

* Printing from Chrome:
(the ToC repeats itself on each printed page)

* Printing from Safari:
(the ToC prints only once on the center page)

ToC and other navigation helpers should better be hidden when printable media is in use, _i.e._
````css
@media print {
/* ... */
}
````

Start with the CSS used by https://opentitan.org/book and inspect the rules controlling the table of contents and other navigation helpers. Add print-media behavior so those helpers are hidden when printing, then verify the result in Chrome and Safari, where the current repetition and centering issues are reported.

Written by the indexing model from the issue text.