Add docs about pre-configuring SSH with pubkey via dietpi.txt

[MichaIng]

as added with DietPi v8.5: MichaIng/DietPi#5509

[StephanStS]

Text contents of the release notes:
Security | It is now possible to add public SSH keys to dietpi.txt, to be applied at first boot for public key authentication with "dietpi" and "root" users. Additionally, SSH password authentication can be disabled for all users or "root" only. Have a look at "AUTO_SETUP_SSH_PUBKEY" and "SOFTWARE_DISABLE_SSH_PASSWORD_LOGINS" settings in /boot/dietpi.txt.

[MichaIng]

And the respective parts in dietpi.txt:

• https://github.com/MichaIng/DietPi/blob/584b862/dietpi.txt#L81-L85
• https://github.com/MichaIng/DietPi/blob/584b862/dietpi.txt#L220-L225

[StephanStS]

Describe also this release notes entry of v8.5
DietPi-Software | Dropbear/OpenSSH
Password authentication is now disabled if set via SOFTWARE_DISABLE_SSH_PASSWORD_LOGINS setting in /boot/dietpi.txt.

[StephanStS]

Docs to be extended there:

• https://dietpi.com/docs/software/ssh/#dropbear
• https://dietpi.com/docs/software/ssh/#openssh
• https://dietpi.com/docs/usage/#how-to-do-an-automatic-base-installation-at-first-boot-dietpi-automation
• https://dietpi.com/docs/install/ : Link to how-to section