Move firebase support into a src directory. (#15)

I'll be adding jekyll support here in a separate commit.

src/README.md

@@ -0,0 +1,11 @@
+<!--
+Part of the Carbon Language, under the Apache License v2.0 with LLVM Exceptions.
+See /LICENSE for license information.
+SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+-->
+
+# Carbon: src
+
+This directory contains tooling related to managing Carbon's documentation.
+
+If you're looking for language tooling, see the `carbon-toolchains` repository.