help-docs: Fix instructions for the "Logging out" mobile app feature.
help-docs: Fix instructions for the "Logging out" mobile app feature.

The instructions for logging out are not accurate.

Fixes the instructions and replaces the iOS and Android tabs with
a single Mobile tab.

Adds a Related articles section with links to "Logging in",
"Switching between organizations", and "Deactivate your account".

rht/zulip,rht/zulip,zulip/zulip,andersk/zulip,andersk/zulip,rht/zulip,rht/zulip,andersk/zulip,zulip/zulip,rht/zulip,zulip/zulip,andersk/zulip,rht/zulip,andersk/zulip,zulip/zulip,zulip/zulip,andersk/zulip,rht/zulip,zulip/zulip,zulip/zulip,andersk/zulip