docs: VS Code integration

[taiki-e]

vscode itself and r-a do not have an API for this, but it appears that using the coverage-gutters extension can make this work.
rust-lang/rust-analyzer#8881 (comment)

If the above way still works well, having an example/guide on this would make sense.

[Th3Whit3Wolf]

I currently do this using llvm-cov nextest --lcov --output-path coverage/lcov.info. Coverage gutters will automatically detect it. Obviously nextest isn't requried and /coverage should be added to .gitignore.