Refined documentation about SecureJoin based on old CounterMitm paper

[hpk42]

In the https://github.com/deltachat/securejoin repository, we started a stripped-down version of the old countermitm-docs. It is available at https://securejoin.delta.chat already and auto-regenerated on commit.

This issue here is about a review and "go-through" in order to a) provide good documentation for the core-rust implementation b) serve as intro-doc for security-auditors. We do not want to rewrite or refactor the docs, but rather want to go for these streamlinings:

• [x] they reflect the current implementation, or we clearly note where they don't
• [ ] link from core-Rust securejoin source code to new securejoin.delta.chat site instead of old countermitm
• [ ] new sub section where we intro and link into core-rust's securejoin sources, referencing #4932 and #4970, and highlighting differences in terminology ("verified" in paper, "protected" in implemetnation, "guaranteed e2ee for users") without attempting to rename or rewrite anything
• [ ] identify missing topics (e.g. Multi-device) and create issues about them in the securejoin repo