Drop dedicated Enum entry pages and remove Enum entries from navigation

[IgnatBeresnev]

Leftover from #2576:

• Enum entries should not have separate pages generated for them
• Enum entries should be removed from the navigation tree

We found little to no evidence for why Enum entries should have separate pages that are dedicated to the entry alone. Most of the time, readers want to be browsing the Enum itself and see which entries it has with their full description (similar to how they're doing it in code).

Notes:

• At the moment, Enum pages contain only brief descriptions of enum entries (so only the first paragraph of the Enum entry's KDoc). Once dedicated pages are removed, it needs to be changed to display full description.

Problems to keep in mind:

• If no pages are generated for enum entries, it could break external documentation links, where a link leads to the enum entry page that does not exist. Moreover, if there's no page, there needs to be a way to link to a specific entry - probably with anchors.
• If enum entries are removed from the navigation tree, auto-scrolling fails with errors as soon as you load an entry's page, as it does not know to which element to scroll to. It needs to be scrolled to the containing enum.