Adds a "Mobile" tab with straightforward instructions for accessing
the long-press menu using two new macros.
Knowing where to access the long-press menu is intuitive, except when
the user is in a topic narrow. It's not immediately obvious that the
top bar can be long-pressed so this adds a "Tip block" using a new macro
to clarify things in this scenario.
Also, this combines the instructions for Desktop and Web into a single
tab because the numbered steps work on both platforms. So this documents
the alternate method via the browser's address bar as a "Tip block" to
avoid stacking alternative numbered steps into a single tab.
This updates the stream and message link instructions too.
Fixes: #22147.