[editorial] Drop 404 link and cleanup references

[chalin]

• Fixes Spec contains an invalid links #169
• Contributes to [CI/infra] Refresh refcache entries for 2024/01 opentelemetry.io#3804
• Drops the 404 link https://pwn.recipes/posts/roll-your-own-e2ee-protocol/. I can't find a reference to this anywhere.
• Cleans up markdown syntax, simplifying the duplication of [https://some-path](https://some-path) by <https://some-path>
• Other copyedits

cc @theletterf @svrnm

[tigrannajaryan]

Thanks!