docs: Update architecture overview introduction.
docs: Update architecture overview introduction.

This doc needs significant further updates, but this at least makes
clear Zulip supports organizations well past 1000 users and adds some
links.

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