Write codex page for Activity admin screens

[DJPaul]

Write a new codex page for the the new Activity admin screens in BP 1.6. It will be linked to from the activity screen's contextual help. Proposed URL is ​http://codex.buddypress.org/buddypress-site-administration/managing-activity/ (if we decide to move this or rename, the URL will need updating in bp-activity-admin.php, or redirected on the codex).

I propose that the new page covers information on how to use the new Activity admin screen (index and edit), as well as detailed info on what core-generated activities look like, e.g. aim to clarify difference between the different activity types use of e.g. primary/secondary item ID, primary link, and the types themselves.

[mercime]

Published the following 3 and 2 years ago respectively:
​https://codex.buddypress.org/administrator-guide/activity-stream-management-panels/
​https://codex.buddypress.org/administrator-guide/activity-streams/
which was combined in
​https://buddypress.org/about/activity/

Note: screenshots will be updated soon.