The comment says that it is called from main.js, but there don't seem to
be any references to it in main.js.

A quick ripgrep says there are no references in all of librustdoc.

Prior to this PR, if the page is restored from the browser bfcache¹, we
call `switchToSavedTheme`. But `switchToSavedTheme` never looks at the
`use-system-theme` preference. Further, if it can't find a saved theme,
it will fall back to the default of "light".

For a user with cookies disabled² whose preferred color scheme is dark,
this means the theme will wobble back and forth between dark and light.
The sequence that occurs is,

1. The page is loaded. During a page load, we consult
   `use-system-theme`: as cookies are disabled, this preference is
   unset. The default is true.

   Because the default is true, we look at the preferred color scheme:
   for our example user, that's "dark". **The page theme is set to
   dark.** We'll attempt to store these preferences in localStorage, but
   fail due to cookies being disabled.

2. The user navigates through the docs. Subsequent page loads happen,
   and the same process in step 1 recurs. Previous pages are
   (potentially) put into the bfcache.

3. The user navigates backwards/forwards, causing a page in bfcache to
   be pulled out of cache. The `pageShow` event handler is triggered.
   However, this calls `switchToSavedTheme`: this doesn't consider the
   system theme, as noted above. Instead, it only looks for a saved
   theme. However, with cookies disabled, there is none. It defaults to
   light. **The page theme is set to light!** The user wonders why the
   dark theme is lost.

There are effectively two functions trying to determine and apply the
correct theme: `updateSystemTheme` and `switchToSavedTheme`. Thus, we
merge them into just one: `updateTheme`. This function contains all the
logic for determining the correct theme, and is called in all
circumstances where we need to set the theme:

* The initial page load
* If the browser preferred color scheme (i.e., light/dark mode) is
  changed
* If the page is restored from bfcache
* If the user updates the theme preferences (i.e., in `settings.js`)

Fixes rust-lang#94250.

¹bfcache: https://web.dev/bfcache/ The bfcache is used to sleep a page,
if the user navigates away from it, and to restore it from cache if the
user returns to it.

²Note that the browser preference that enables/disables cookies really
controls many forms of storage. The same preference thus also affects
localStorage. (This is so a normal browser user doesn't need to
understand the distinction between "cookies" and "localStorage".)