docs/scanrefs: fix handling if ref is same as headline

docs/scanrefs: fix handling if ref is same as headline

If the ref is named the same as the headline (once normalized), sphinx
will return a 'idX' value in node['ids'][1] which we use for the label
ID. The headline is always present at index 0.

Checking for that and using index 0 in case we do get a 'idX' helps us
to avoid using the 'idX' as keys in our OnlineHelpInfo.js and actually
use the intended key.

Signed-off-by: Aaron Lauterer <a.lauterer@proxmox.com>