Introduce the idea of a Buck "project" distinct from a repo
Introduce the idea of a Buck "project" distinct from a repo

Summary:
- Add `.buckconfig` to this page as a fundamental concept for understanding Buck.
- Call out that `.buckconfig` defines the root of the Buck //project//.
- Clarify that a single repo could have multiple Buck projects each with their own `.buckconfig`.

Reviewed By: ttsugriy

fbshipit-source-id: 6086baf5aa

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