Improve arcanist install documentation

Summary:
See https://github.com/facebook/arcanist/issues/21, https://github.com/facebook/arcanist/issues/22.

• Provide more detailed install instructions.
• Provide advice on managing team installs.
• Provide information on libphutil path resolution.
• Provide OSX-specific documentation.
• Update some of the documentation to reflect evolution of the tools.

Test Plan: Generated and read documentation.

Reviewers: btrahan

Reviewed By: btrahan

CC: jmhsieh, aran, epriestley

Differential Revision: https://secure.phabricator.com/D1990