[FAB-2729] (PA) Document the configtx

https://jira.hyperledger.org/browse/FAB-2729

This CR introduces a new configtx.md document to fabric/docs. It
describes the technical details of the configtx implementation, the
flows for how configurationis updated, and how channel creation is
performed and authorized.

convert to rst format, move to docs/source directory

Change-Id: If96d670f0d33ffe66109226f38cc3e824a145762
Signed-off-by: Jason Yellick <jyellick@us.ibm.com>
Signed-off-by: Nick Gaski <ngaski@us.ibm.com>