pw_build: Python build docs

- Document Python build automation as part of the general build system
  docs.
- Document the Python GN templates in pw_build.
- Add links between relevant sections of docs.

Change-Id: I80dfbdf0e49a63eab232895afb91d05d95015613
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/33781
Reviewed-by: Keir Mierle <keir@google.com>
Reviewed-by: Joe Ethier <jethier@google.com>
Reviewed-by: David Rogers <davidrogers@google.com>
Commit-Queue: Wyatt Hepler <hepler@google.com>
diff --git a/pw_build/BUILD.gn b/pw_build/BUILD.gn
index ab0f9f5..7668dd0 100644
--- a/pw_build/BUILD.gn
+++ b/pw_build/BUILD.gn
@@ -134,7 +134,10 @@
 }
 
 pw_doc_group("docs") {
-  sources = [ "docs.rst" ]
+  sources = [
+    "docs.rst",
+    "python.rst",
+  ]
 }
 
 # Pool to limit a single thread to download external Go packages at a time.