Update installation docs for current Known UI

Add more explicit instructions for setting up the instance-level
Twitter support through the "Site configuration" menu before
users will be able to connect to Twitter. This will help avoid
the currently raw error that gets thrown when the Twitter API
keys have not yet been configured.

Use "Known" to refer to the software instead of "idno".

Signed-off-by: Dan Scott <dan@coffeecode.net>