prod docs: Update BBB configuration link.
prod docs: Update BBB configuration link.

Updates the Big Blue Button customization link for
extracting shared secrets.

kou/zulip,andersk/zulip,andersk/zulip,zulip/zulip,eeshangarg/zulip,rht/zulip,andersk/zulip,andersk/zulip,andersk/zulip,kou/zulip,kou/zulip,andersk/zulip,kou/zulip,rht/zulip,eeshangarg/zulip,zulip/zulip,eeshangarg/zulip,zulip/zulip,zulip/zulip,andersk/zulip,eeshangarg/zulip,eeshangarg/zulip,rht/zulip,eeshangarg/zulip,zulip/zulip,rht/zulip,kou/zulip,rht/zulip,eeshangarg/zulip,kou/zulip,zulip/zulip,kou/zulip,zulip/zulip,rht/zulip,rht/zulip