[docs] Use RTD links for OPNFV Fuel docs refs

[docs] Use RTD links for OPNFV Fuel docs refs

Now that OPNFV Docs has moved away from git submodules and each
project gets its own RTD page, intersphinx labels are broken when
reading `opnfv-armband` documentation individually.

Switch to using explicit https links to RTD, so all links are working
no matter the way documentation is accessed. This has the obvious
downside of having to maintain those links in case they ever change
upstream.

Change-Id: I4945f51c1c5710c57da3dfdb8adda30bb283b30a
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>