This change moves the RST DLO documentation from
statically inside overview_large_objects.rst and moves it
to middleware/dlo.py.
This is where all middleware RST documentation is defined.
The overview_large_objects.rst is still the main page
for information on large objects, so now dynamically
points to both the DLO and SLO middleware RST
documentation and the relevant middleware.rst page
simply points to it.
Change-Id: I40d918c8b7bc608ab945805d69fe359521df038a
Closes-bug: #1276852
384b91eb82