docs: move user and interface .tex documents under reference.

docs: move user and interface .tex documents under reference.

Taking over the top level "user" entry with a relatively obsolete document is a
bit of an annoyance but these docs are not so out of date that they should be
deleted. Move them out of the top-level instead.

(the original motivation here was to allow for user/xl-domain-cfg.markdown but
we have since decided to go with man/xl.cfg.pod.5 instead so perhaps this is a
waste of time)

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Jackson <ian.jackson.citrix.com>
Acked-by: Ian Jackson <ian.jackson.citrix.com>