Wireguard - S2S Configuration unclear + MTU/MSS not explained

[Monviech]

Important notices

Before you add a new report, we ask you kindly to acknowledge the following:

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

Describe the issue

MTU/MSS issued with Wireguard arent hinted in the doc: https://forum.opnsense.org/index.php?topic=35877

Wireguard Site 2 Site configuration is unclear in the doc: https://forum.opnsense.org/index.php?topic=35486

Suggestions

I rewrote the Wireguard S2S doc completely and improved the Wireguard client doc. Both have Normalization for MSS included. The S2S docs now guides the user step by step through an example.

PR: https://github.com/opnsense/docs/pull/498