Draft: docs: Add a "Getting Started" guide to the website

• Divide the "Achieving deterministic builds" section under "Documentation" into two distinct categories: "Achieving deterministic builds" (what you should do as a developer of reproducible code) and "Managing variance" (what to do as a developer/maintainer when variance starts to cause problems for builds of software you care about).

• Provide a diagram to indicate what developers can and should do to prove to themselves and others that their software is reproducible. Start simple; just rebuild it and check -- but add as many variance factors as the community is aware of to really gain confidence that the software builds to a repeatable, deterministic result.

  • Related: add a troubleshooting guide page -- some common examples of what to do when you discover that a software project does not build reproducibly, and some pattern-matching techniques to figure out what to do about it.

  • Related: add a page describing how to add variance in a systematic way; tooling available to do so, etc.

Resolves #56