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