Revise the first two chapters, bringing them up to release 5.14.2 (20.04.2).

Add more pictures, and remove all locally defined entities from index.docbook. Compressed the images used in the first two chapters to 8-bit color using pngquant. IMPORTANT NOTE: I have recently updated "contributor.entities" and "general.entities" in the central "kdoctools" repository (src/customization/entities/), so these .docbook files (index, intro, and getting-started) will work locally if, and only if, you update those two files in /usr/share/kf5/kdoctools/customization/entities/.