Merge #8274

8274: Adding a few more gifs and screenshots for features in manual r=Veykril a=MozarellaMan

Related  #8267,#6539. Gifs are [here](https://github.com/rust-analyzer/rust-analyzer/issues/6539#issuecomment-809574840)

Finishing up the last PR,  for the last two features that didn't have a visual example.

For syntax highlighting, I wasn't able to find a theme that displayed the difference between an enum and struct, but I only tried a few apart from the default so there could be one out there!

e.g., with the default light theme, `Ord` and `Ordering` in `use std::cmp::{Ord, Ordering}` had the same highlight colour. So I just went with displaying `mut` items being underlined.

Co-authored-by: Ayomide Bamidele <48062697+MozarellaMan@users.noreply.github.com>