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_bloat/bloat.rst b/pw_bloat/bloat.rst
index b6448b7..73003a1 100644
--- a/pw_bloat/bloat.rst
+++ b/pw_bloat/bloat.rst
@@ -4,9 +4,9 @@
 
 .. highlight:: sh
 
------
-Bloat
------
+--------
+pw_bloat
+--------
 The bloat module provides tools to generate size report cards for output
 binaries.