pw_docgen: Fix paths, multiple sources

- Fix code for remapping paths into the docgen tree. Some path prefixes
  were being incorrectly trimmed, preventing size report inclusion in
  some cases.
- Make Sphinx warnings errors.
- Allow specifying multiple top-level sources to a pw_doc_gen.
- Move documentation for modules to their own table of contents.
- Rename module documentation to the module names.

Change-Id: Ic475019673ccefdb786a28d241a143886af097ab
diff --git a/docs/BUILD.gn b/docs/BUILD.gn
index b45bfe8..0dffb53 100644
--- a/docs/BUILD.gn
+++ b/docs/BUILD.gn
@@ -23,7 +23,10 @@
 
 pw_doc_gen("docs") {
   conf = "conf.py"
-  index = "index.rst"
+  sources = [
+    "index.rst",
+    "modules.rst",
+  ]
   output_directory = target_gen_dir
   deps = [
     ":core_docs",