New source: use Navi cheatsheats

[denisidoro/navi](https://github.com/denisidoro/navi) is a trending cheatsheet viewer/manager that uses its own cheatsheets along with custom user-made ones. It uses [this cheatsheet repo](https://github.com/denisidoro/cheats) by default. Other repositories exist, some of which are listed in its [`featured_repos.txt`](https://github.com/denisidoro/cheats/blob/master/featured_repos.txt) file.

The file format is described in Navi's [README](https://github.com/denisidoro/navi/blob/master/README.md#cheatsheet-syntax).

Start by reading Navi's README section on cheatsheet syntax and its featured_repos.txt file to understand the source format and available repositories. Then locate how cheat.sh adds external cheatsheet sources. Done means Navi's cheatsheets are available through the service and the relevant behavior is covered by existing checks, if present.

Written by the indexing model from the issue text.