doc/_static: match with branch encoded in pathname in URL

doc/_static: match with branch encoded in pathname in URL

since we've migrated to RTD, a typical URL of ceph document looks like

https://docs.ceph.com/en/octopus/rados/

so instead of match "/docs.ceph.com/docs/<branch-name>/", we should
better match "/en/<branch-name>". in this change,

* do not include the domain name in the pattern to be matched,
  it's not necessary.
* match with "/en/<branch-name>/"

Signed-off-by: Kefu Chai <kchai@redhat.com>