Update doc wrt. Liferea → Thunderbird as the default feed reader

At least:

• drop Liferea from the list of features
• add feed reading to the list of Thunderbird features on doc/about/features

Feature Branch: doc/15745-deprecate-liferea

Parent Task: #11082 (closed)

Related issues

• Blocked by #15091 (closed)
• Blocks #15411 (closed)
• Blocks #15776 (closed)

Original created by @intrigeri on 15745 (Redmine)