Expand build system docs

This updates the GN build system's documentation to more generally
describe how to configure a Pigweed project using GN, instead of
specifically explaining the upstream build structure.

Change-Id: Ic3d9423a34400ccfd57da8f642ced7721fe835e4
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/17402
Commit-Queue: Alexei Frolov <frolv@google.com>
Reviewed-by: Keir Mierle <keir@google.com>
diff --git a/pw_build/docs.rst b/pw_build/docs.rst
index 16e9d9f..9fa2944 100644
--- a/pw_build/docs.rst
+++ b/pw_build/docs.rst
@@ -79,6 +79,8 @@
 All of the ``pw_*`` target type overrides accept any arguments, as they simply
 forward them through to the underlying target.
 
+.. _pw-build-python-script:
+
 pw_python_script
 ^^^^^^^^^^^^^^^^
 The ``pw_python_script`` template is a convenience wrapper around ``action`` for