[rbowen]

Closes #358

The Binary tab in the installation guide had two port-related problems:

1. The example used INSTALL_PORT=80. On Linux, ports below 1024 require
   root, so this fails for a normal user.
2. Nothing explained that INSTALL_PORT only sets the installer's port.
   After installation, Answer listens on server.http.addr in
   answer-data/conf/config.yaml (default 0.0.0.0:80), regardless of the
   port used for the installer. Users who installed on another port find
   that ./answer run tries to bind port 80.

This PR:

• Changes the example to INSTALL_PORT=9080 (matching the Docker examples)
  and updates the install URL to match
• Adds a note explaining server.http.addr and when to change it

Only the Binary tab of docs/getting-started/installation.md is changed.