Buildkit - Improve buildkit page

- General formatting improvements.
- Change  "Ubuntu 12.04 or 14.04" to "Ubuntu 12.04 or later" because the
  buildkit install script is clearly designed to work for later versions
  of Ubuntu.
- Relocate troubleshoot steps to after configuration steps (so the
  reader doesn't stop reading when they see "troubleshooting".
- Higlight post-installation steps.
- New instructions for manually ading webserver settings after running
  `amp config`.
- More troubleshooting help.