Add CODEBASE.md, a description of the codebase
Add CODEBASE.md, a description of the codebase

The content was created by lberki@ and discussed here: https://docs.google.com/document/d/1wzjTCuqXNKBlEEHCVw17j7kil2qx04sBsMZCTZzOJj0/edit
It was converted automatically to Markdown. I only added the 3 lines at the top of the page.

Closes https://github.com/bazelbuild/bazel/pull/11923

RELNOTES: Added CODEBASE.md, a description of the Bazel codebase.
PiperOrigin-RevId: 326276188

cushon/bazel,bazelbuild/bazel,cushon/bazel,twitter-forks/bazel,perezd/bazel,davidzchen/bazel,werkt/bazel,ButterflyNetwork/bazel,meteorcloudy/bazel,meteorcloudy/bazel,davidzchen/bazel,cushon/bazel,cushon/bazel,twitter-forks/bazel,safarmer/bazel,safarmer/bazel,ButterflyNetwork/bazel,cushon/bazel,katre/bazel,twitter-forks/bazel,davidzchen/bazel,twitter-forks/bazel,ButterflyNetwork/bazel,bazelbuild/bazel,bazelbuild/bazel,katre/bazel,katre/bazel,perezd/bazel,perezd/bazel,ButterflyNetwork/bazel,safarmer/bazel,safarmer/bazel,bazelbuild/bazel,twitter-forks/bazel,davidzchen/bazel,werkt/bazel,perezd/bazel,ButterflyNetwork/bazel,perezd/bazel,bazelbuild/bazel,twitter-forks/bazel,werkt/bazel,davidzchen/bazel,werkt/bazel,werkt/bazel,meteorcloudy/bazel,meteorcloudy/bazel,safarmer/bazel,katre/bazel,davidzchen/bazel,safarmer/bazel,meteorcloudy/bazel,meteorcloudy/bazel,perezd/bazel,katre/bazel,twitter-forks/bazel,meteorcloudy/bazel,perezd/bazel,davidzchen/bazel,ButterflyNetwork/bazel,bazelbuild/bazel,katre/bazel,cushon/bazel,werkt/bazel