Utilities/Sphinx: Index guide section documents for cross-referencing

Extend the change from commit 6185265800 (Utilities/Sphinx: Index
guide-level documents for cross-referencing, 2019-12-06,
v3.17.0-rc1~181^2~1) to name guide documents other than the top level
using their docname instead of their title.  This will allow them to be
cross-referenced by a stable name even if their title changes.