8267: Adding gifs and screenshots for features in manual r=matklad a=MozarellaMan
For #6539
This includes most of gif or screenshot examples of most items in the "Features" header. With the exceptions of:
- **On Typing Assists** - couldn't get it to work for a demo, I'm probably missing something?
- **Structural search and replace** - looked to be already a visual example of the feature
- **Workspace symbol** - wasn't sure how best to show this, all of the examples maybe? Also wasn't sure of the best code example to show it off
- **Semantic Syntax Highlighting** - seemed obvious enough to not need a screenshot, but I could easily add this
All the gifs/pngs are hosted in this [comment](https://github.com/rust-analyzer/rust-analyzer/issues/6539#issuecomment-809574840). Please let me know if any of them aren't suitable (and why) and I'll improve it! Or if you don't like the theme/font
Co-authored-by: Ayomide Bamidele <48062697+MozarellaMan@users.noreply.github.com>
