doc: Remove ".vnet" from paulmck email addresses

Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>

docs: core-api: add circular-buffers documentation

The circular-buffers.txt is already in ReST format. So, move it to the
core-api guide, where it belongs.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>