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>
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