Docs: Update configuration docs

Update configuration docs to align with changes which decouples option
configuration and validation in config header files.

Convert header file config system diagrams in svg format
It is difficult to modify existing figures in header_file_system.rst and
kconfig_header_file_system.rst.
Re-draw those figures in svg format and enable developers to more easily
update the diagram.

Change-Id: If88a6a07104ef54de127ccb9149c5ff4792b3cdd
Signed-off-by: David Hu <david.hu@arm.com>