Documentation should be built with sphinx, and installed. Linking the documentation with the sage documentation is an unsolved problem - see the following tickets

• #27697 (Make pplpy doc location configurable) – https://trac.sagemath.org/ticket/27697
• #27495 (links to external docs in main reference index.rst) –https://trac.sagemath.org/ticket/27495#comment:40
• #27164 (link sphinx docs of Sage components in reference manual) – Sage
  https://trac.sagemath.org/ticket/27164

This issue is not very important because the current documentation is not very useful anyway. The doctests should actually be migrated to _test... methods of GenericBackend.