R1:968273445908

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

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