gpgv: refresh page and add example

[FazleArefin]

Checklist

• The page(s) are in the correct platform directories: common, linux, osx, windows, sunos, android, etc.
• The page description(s) have links to documentation or a homepage.
• The page(s) follow the content guidelines.
• The page(s) follow the style guide.
• The PR contains at most 5 new pages.
• The PR is authored by me, or has been human-reviewed if it was created with AI or machine translation software.
• The PR title conforms to the recommended templates.
• Version of the command being documented (if known): 2.4.7

Reference issue: #

[FazleArefin]

Note that for the new example the command is long and using a intermediary file (path/to/pubkey.gpg) because the --keyring option in gpgv does not work with named pipes.

gpg --dearmor {{[-o|--output]}} {{path/to/pubkey.gpg}} < {{path/to/pubkey.txt}} && gpgv --keyring {{path/to/pubkey.gpg}} {{path/to/signature.asc}} {{path/to/data_file}}