user docs: Allow redirection of external URLs in documentation sidebars.
user docs: Allow redirection of external URLs in documentation sidebars.

Fixes #7916.

hackerkid/zulip,showell/zulip,synicalsyntax/zulip,dhcrzf/zulip,punchagan/zulip,punchagan/zulip,kou/zulip,rishig/zulip,rishig/zulip,rht/zulip,hackerkid/zulip,rht/zulip,brainwane/zulip,brainwane/zulip,rht/zulip,jackrzhang/zulip,tommyip/zulip,zulip/zulip,shubhamdhama/zulip,zulip/zulip,rishig/zulip,andersk/zulip,kou/zulip,zulip/zulip,shubhamdhama/zulip,kou/zulip,andersk/zulip,dhcrzf/zulip,andersk/zulip,timabbott/zulip,kou/zulip,shubhamdhama/zulip,jackrzhang/zulip,hackerkid/zulip,showell/zulip,rishig/zulip,brainwane/zulip,timabbott/zulip,synicalsyntax/zulip,eeshangarg/zulip,hackerkid/zulip,tommyip/zulip,andersk/zulip,shubhamdhama/zulip,jackrzhang/zulip,eeshangarg/zulip,tommyip/zulip,kou/zulip,zulip/zulip,dhcrzf/zulip,synicalsyntax/zulip,brainwane/zulip,brainwane/zulip,timabbott/zulip,rht/zulip,timabbott/zulip,showell/zulip,eeshangarg/zulip,punchagan/zulip,eeshangarg/zulip,tommyip/zulip,showell/zulip,punchagan/zulip,showell/zulip,zulip/zulip,shubhamdhama/zulip,eeshangarg/zulip,punchagan/zulip,kou/zulip,rishig/zulip,eeshangarg/zulip,kou/zulip,synicalsyntax/zulip,rishig/zulip,dhcrzf/zulip,rht/zulip,zulip/zulip,jackrzhang/zulip,andersk/zulip,synicalsyntax/zulip,jackrzhang/zulip,hackerkid/zulip,synicalsyntax/zulip,rishig/zulip,dhcrzf/zulip,rht/zulip,hackerkid/zulip,punchagan/zulip,jackrzhang/zulip,dhcrzf/zulip,tommyip/zulip,andersk/zulip,punchagan/zulip,synicalsyntax/zulip,shubhamdhama/zulip,rht/zulip,andersk/zulip,timabbott/zulip,tommyip/zulip,dhcrzf/zulip,timabbott/zulip,tommyip/zulip,shubhamdhama/zulip,timabbott/zulip,hackerkid/zulip,showell/zulip,brainwane/zulip,brainwane/zulip,eeshangarg/zulip,showell/zulip,jackrzhang/zulip,zulip/zulip