Update Icedove documentation

There was Thunderbird documentation that still hangs around, but needs to be updated:
https://tails.boum.org/doc/anonymous_internet/thunderbird/index.en.html

• Explain briefly the benefits of an email client over webmail in the context of Tails:
  • Faster interactions
  • Easier to do OpenPGP

Also, it’s likely that some Tails-specific aspects need to be documented, as a result of e.g. #6154 (closed) and #6151 (closed).

Feature Branch: doc/7158-icedove

Parent Task: #5663 (closed)

Original created by @BitingBird on 7158 (Redmine)