user docs: Organize message sending and reading docs.
user docs: Organize message sending and reading docs.

Collects all relevant docs into "Sending Messages" and "Reading Messages".

Doesn't add or remove any docs, just moves them around on the sidebar.
Duplicates "Message a stream by email" (it now appears in two sections)
Removes the "Editing Messages" section/header.

dhcrzf/zulip,jackrzhang/zulip,timabbott/zulip,jackrzhang/zulip,punchagan/zulip,punchagan/zulip,tommyip/zulip,rishig/zulip,dhcrzf/zulip,mahim97/zulip,brainwane/zulip,showell/zulip,synicalsyntax/zulip,zulip/zulip,hackerkid/zulip,synicalsyntax/zulip,rishig/zulip,shubhamdhama/zulip,timabbott/zulip,kou/zulip,timabbott/zulip,Galexrt/zulip,brainwane/zulip,Galexrt/zulip,rishig/zulip,eeshangarg/zulip,zulip/zulip,brainwane/zulip,mahim97/zulip,showell/zulip,rht/zulip,jackrzhang/zulip,punchagan/zulip,zulip/zulip,hackerkid/zulip,showell/zulip,rht/zulip,punchagan/zulip,brockwhittaker/zulip,shubhamdhama/zulip,andersk/zulip,tommyip/zulip,brainwane/zulip,showell/zulip,dhcrzf/zulip,brainwane/zulip,brockwhittaker/zulip,kou/zulip,tommyip/zulip,Galexrt/zulip,dhcrzf/zulip,andersk/zulip,zulip/zulip,Galexrt/zulip,rishig/zulip,jackrzhang/zulip,showell/zulip,andersk/zulip,hackerkid/zulip,zulip/zulip,dhcrzf/zulip,eeshangarg/zulip,kou/zulip,rishig/zulip,dhcrzf/zulip,brainwane/zulip,shubhamdhama/zulip,timabbott/zulip,andersk/zulip,rht/zulip,shubhamdhama/zulip,rht/zulip,jackrzhang/zulip,mahim97/zulip,hackerkid/zulip,rht/zulip,eeshangarg/zulip,synicalsyntax/zulip,tommyip/zulip,kou/zulip,eeshangarg/zulip,punchagan/zulip,dhcrzf/zulip,tommyip/zulip,punchagan/zulip,brockwhittaker/zulip,kou/zulip,Galexrt/zulip,mahim97/zulip,jackrzhang/zulip,rishig/zulip,eeshangarg/zulip,brockwhittaker/zulip,shubhamdhama/zulip,andersk/zulip,brockwhittaker/zulip,kou/zulip,zulip/zulip,hackerkid/zulip,tommyip/zulip,Galexrt/zulip,hackerkid/zulip,timabbott/zulip,eeshangarg/zulip,synicalsyntax/zulip,timabbott/zulip,timabbott/zulip,synicalsyntax/zulip,brainwane/zulip,jackrzhang/zulip,andersk/zulip,Galexrt/zulip,showell/zulip,punchagan/zulip,synicalsyntax/zulip,rishig/zulip,showell/zulip,rht/zulip,shubhamdhama/zulip,shubhamdhama/zulip,zulip/zulip,mahim97/zulip,rht/zulip,brockwhittaker/zulip,kou/zulip,mahim97/zulip,synicalsyntax/zulip,andersk/zulip,hackerkid/zulip,tommyip/zulip,eeshangarg/zulip