[Add]: Members component explanation

[imath]

What needs to be done?

It should better explain than this outdated Codex page what comes with this component and show generated front-end areas (directory, single item) as well as tell some words about features such as member types or membership invites.

Finally, it should link to other components integration with Members component instead of duplicating informations like it‘s currently the case on the Codex page.

Handbook to add the page in

buddypress/docs/user

Tasks to accomplish

• This task has been added to the "Todo" panel of the Documentation project.
• A PR has been submitted to the BuddyPress GH repository, it has been linked to this issue/task & the task was moved to the "In progress" panel of the Documentation project.
• The PR has been converted to a SVN patch and committed to the BuddyPress SVN repository & the task was moved to the "Done" panel of the Documentation project.