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/pw_preprocessor/docs.rst b/pw_preprocessor/docs.rst
index 37ddbf6..048abb3 100644
--- a/pw_preprocessor/docs.rst
+++ b/pw_preprocessor/docs.rst
@@ -4,9 +4,9 @@
 
 .. highlight:: sh
 
-------------
-Preprocessor
-------------
+---------------
+pw_preprocessor
+---------------
 The preprocessor module provides various helpful preprocessor macros.
 
 Compatibility