move contrib plugin directives out of ikiwiki/directive

Problem is that ikiwiki/directive is turned into an underlay and added to
all wikis. And the test suite checks it for broken links and turns some up.
So it's not appropriate for contrib plugin's directives to be documented
in there.

diff --git a/doc/ikiwiki/directive/ftemplate.mdwn b/doc/plugins/contrib/ftemplate/ikiwiki/directive/ftemplate.mdwn
similarity index 100%
rename from doc/ikiwiki/directive/ftemplate.mdwn
rename to doc/plugins/contrib/ftemplate/ikiwiki/directive/ftemplate.mdwn

diff --git a/doc/ikiwiki/directive/report.mdwn b/doc/plugins/contrib/report/ikiwiki/directive/report.mdwn
similarity index 100%
rename from doc/ikiwiki/directive/report.mdwn
rename to doc/plugins/contrib/report/ikiwiki/directive/report.mdwn