Rework /doc/about/anonymity/

This piece of doc is still inherited from Incognito and hasn’t moved since then.

- What do we want to do with it?

- At least we should update the reference to ECHELON to something newer.
- Shall we update the external links?

In more general terms, this piece of documentation is not specific to Tails, but about general security topics that we usually don’t deal with in our documentation.

Parent Task: #5977 (closed)

Related issues

• Related to #5816 (closed)

Original created by @sajolida on 7139 (Redmine)