[Add]: BP REST API v2

[imath]

What needs to be done?

The https://developer.buddypress.org/bp-rest-api/ site will remain the BP REST API v1 documentation site. We need to take the existing content, update it according to changes introduced in v2 and host it in BuddyPress repo's docs/developer part.

Handbook to add the page in

buddypress/docs/developer

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.