We add

src/sage_docbuild/ext/inventory_builder.py
src/sage_docbuild/ext/multidocs.py
src/sage_docbuild/ext/sage_autodoc.py

to the doc of sage documentation system.

Along the way, we also edit the top level index page appropriately.

Component: documentation

Author: Kwankyu Lee

Branch/Commit: 48ba8d0

Reviewer: Matthias Koeppe

Issue created by migration from https://trac.sagemath.org/ticket/34642