New introduction for Platform Manual

[Andrew Eddie]

I've added a new introduction to the Platform Manual which goes over
bootstrapping and the JPlatform and JLoader classes.

http://developer.joomla.org/platform-manual.html

Enjoy (and watch for typo's) and/or ask questions so I can improve
that content :)

I've also reformatted the manual a bit so that the core packages are
all in one chapter, and also the change log is now just a version
history since we have the exhaustive change report on the developer
site for each version.

Regards,
Andrew Eddie
http://learn.theartofjoomla.com - training videos for Joomla developers