Interfaces: Devices: Point to Point section needs a mention in the devices manual page

[fichtner]

Important notices

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• I have read the contributing guide lines at https://github.com/opnsense/docs/blob/master/CONTRIBUTING.md
• I have searched the existing issues and I am convinced that mine is new.

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

The removal of 9ed75e3 showed that we don't actually have a page describing the PPP settings. The access of PPP settings through the interfaces settings has been removed to make the code more maintainable.

Describe the solution you like

• Add a Point to Point section with the most useful settings removed in 9ed75e3 plus some more as these could be important to users.
• Also add a tutorial for PPPoE setup from scratch via the GUI (not the wizard) which is something we are definitely missing as well.

Describe alternatives you considered

N/A

Additional context

#643