STYLE: Add README markdown files for each folder

Documentation will gradually be added to the markdown files, but for now they contain a title and a generic footer. Also created a Base/Common folder, into which some of the files from Utilities will be added, per discussion in meeting.

ENH: Update Cppcheck suppressions
ENH: Suppress CTK manually-specified variables CMake warning

Change-Id: I43196f8ff70666a1f315be2b90bce42441db8032