docs: update Mesh VPN documentation

- Restructure page
- Add information on how to add L2TPv3 offloading support to a build
  using configurable ciphers. The null method is not reocmmended anymore.
- Add notes and pointers regarding the gateway configuration to provide
  gateway admins with hints on how to modify their configuration to
  accommodate this new feature.
- Mention wireguard support

Based-on-patch-by: Felix Kaechele <felix@kaechele.ca>