closes freifunk-gluon/gluon#2603

Co-authored-by: Martin Weinelt <martin@darmstadt.freifunk.net>

- 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>

2 spaces is the most common indentation width used in the docs; adjust
the rest for consistency.

Also change .editorconfig accordingly.