docs: Fix broken user docs links in new feature tutorial.
docs: Fix broken user docs links in new feature tutorial.

Surprisingly hard to find a good base example of a user doc. This one should
at least give the basic pointer to sidebar_index.md and where the new file
goes.

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