Use Tor Browser for offline documentation and drop the documentation browser

As per #15612-note_5, now that #14962 (closed) will be fixed via #15023 (closed).

Feature Branch: feature/15720-drop-custom-doc-viewer+force-all-tests

Related issues

• Related to #15612 (closed)
• Blocked by #15023 (closed)
• Blocks #15334 (closed)

Original created by @intrigeri on 15720 (Redmine)