Add manpage that linux distributions can ship

Generally I expect a binary like `/usr/bin/cabal` to be accompanied by a helpful manpage.
This should include available flags/subcommands and perhaps a few examples.

Start from the /usr/bin/cabal command and inventory the available flags and subcommands that the manpage should cover. Define the manpage content, including a few examples, and verify that the result is suitable for Linux distributions to ship.

Written by the indexing model from the issue text.