docs/images: add gifs and images for features

This includes images and gifs for:
- add import
- add tags to struct fields
- call hierarchy
- completion signature help
- extract variable
- find all references
- generate interface implementation
- generate unit tests
- go to definition
- find implementations
- outline
- peek definition
- rename
- toggle test file
- upgrade dependencies
- workspace symbols

Change-Id: I32394891ba1ecbeef87d0a5f4b11be969675c0a9
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/284272
Trust: Suzy Mueller <suzmue@golang.org>
Run-TryBot: Suzy Mueller <suzmue@golang.org>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>

Author: suzmue