Merge pull request #46 from groessler/gh-pages_to_pull

Documentation changes for Atari XL target.
Removed empty HTML table cells.
Opt-out of Jekyll processing.
Add information about "system check" load chunk (now also in the regular atari target).
Add information about problems when upgrading (MAINHDR).
Add information about to get rid of "system check" load chunk.
Move "Technical details" in front of "Other hints".