Proofread XCF format specification.

Add outline, correct headlines.
Chapter 1: rearrange, add URL to Cinepaint XCF format spec, wording.
Replace 'Gimp' by 'GIMP', 'the GIMP' by 'GIMP'.
Fix typo RIB->RGB, GIMP 3.3+, rearrange headline.
Clarify: 'XCF' files from other software.
Correct spelling.
Remove trailing whitespaces.