docs: Add overview of the structure of vsperf code

Text & sequence diagrams as both source and png.

JIRA: VSPERF-106

Signed-off-by: Billy O'Mahony<billy.o.mahony@intel.com>
Reviewed-by: Al Morton <acmorton@att.com>
Reviewed-by: Eugene Snider <Eugene.Snider@huawei.com>
Reviewed-by: Maryam Tahhan <maryam.tahhan@intel.com>
Reviewed-by: Gurpreet Singh <gurpreet.singh@spirent.com>
Reviewed-by: Brian Castelli <brian.castelli@spirent.com>
Change-Id: I33f57fba85710ab4cfc72d0a5d06db6f653545d6