fix small issues in formatting

* fix warning from "rst2html --verbose" by not using the in-line hyperlink
  syntax. The warning said::

    known_issues.rst:261: (INFO/1) Hyperlink target "google chart api" is not referenced.

* remove dead code (no-longer-referenced link), thus fixing another warning

* change links to relative files to be just $FILENAME instead of
  file:$FILENAME, which I think will fix the problem of broken links on the
  trac and github, e.g. the link in
  https://tahoe-lafs.org/trac/tahoe-lafs/browser/trunk/docs/known_issues.rst ,
  which ought to go to
  https://tahoe-lafs.org/trac/tahoe-lafs/browser/trunk/docs/cautions.rst , but
  instead goes to file:///cautions.rst