WL#3435: PHP: ext/mysql -> ext/mysqli converter tool -- article on architecture/developing

Write an article which describes the software architecture of the converter tool
and teaches users how to a) expand the software and b) identify bugs.

It's planned to use an article series instead of classical documentation to
describe the tool. The advantage of articles is that we can them for marketing
purposes. Classical documentation seems not needed as the usage of the tool
itself has to be made self-explaining.