Fix #7712 fix #7803: do not link to other .rst file by .html links anymore

Links to .html files are not checked by sphinx and thus bit-rot.
Rather, link by :ref:, :doc:, :option: etc.

This commit does not address the links of the form

  ../release/cabal-latest/doc/API/**.html

which I do not know how to handle.  These are all broken on
readthedocs.org and I have no clue how these were supposed to work.