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/index.rst b/docs/index.rst
index 5b2d715..055690e 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -8,11 +8,9 @@
 facilitate easy integration into existing codebases.
 
 .. toctree::
-   :maxdepth: 1
-   :hidden:
+  :maxdepth: 1
+  :hidden:
 
-   docs/embedded_cpp_guide
-   docs/style_guide
-   pw_bloat/bloat
-   pw_docgen/docgen
-   pw_preprocessor/docs
+  docs/embedded_cpp_guide
+  docs/style_guide
+  modules