Show fingerprint of the SSH host key

[FranklinYu]

Feature Description

GitLab automatically shows the fingerprint under path /help/instance_configuration#ssh-host-keys-fingerprints, such as this page on the official instance. It would be nice for Gitea to also publish this information automatically.

This is useful when connecting to Gitea the first time, and users will be prompted something like

The authenticity of host 'git.example.com (192.0.2.1)' can't be established.
ECDSA key fingerprint is SHA256:HbW3g8zUjNSksFbqTiUWPWg2Bq1x8xdGUrliXFzSnUw.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added 'git.example.com' (ECDSA) to the list of known hosts

Right now users can only blindly trust it, leaving room for potentially MitM attack.

Screenshots

No response

[FranklinYu]

Since Gitea doesn’t bundle documentation into the installation, we might have to add a new route specific for this?