Dogfooding: Create a PR on bazel.build explaining the bazel.online flow

## Description
Once we announce this at Bazelcon, it will be crucial to explain what are the steps to follow to make contributions and what the contributors should expect to see. The best way is creating documentation directly on bazel.build that goes through the pipeline and gets generated on bazel.online. That way we also dogfood the process.

## Definition of done
This page should include:

1. an architecture diagram
2. Explanation of the flow
3. Where should people add content to (bazel.build)
4. Where they should expect to see the results for the PR (bazel.online)