Manual page (manpage) for Man-db

### Is your feature request related to a problem? Please describe.

lazygit is a terminal-based user interface.
Users of Linux distributions often rely on offline manual pages (man), but no official manpage currently exists for lazygit.

### Describe the solution you'd like

Add and maintain an official manpage within the lazygit project.

### Describe alternatives you've considered

lazygit already provides well-written and well-maintained web documentation:

* https://github.com/jesseduffield/lazygit/blob/master/README.md
* https://github.com/jesseduffield/lazygit/blob/master/docs/README.md

These documents are also included in the Debian package:

* https://packages.debian.org/sid/amd64/lazygit/filelist

However, they are not formatted for traditional command-line manual page interfaces.