Changes history

(Only outstanding content changes listed, the texts and code are constantly updated and fixed thanks to awesome contributors.)

2019-10-14

2.4 changelog added;
2.5 and 2.6 updated significantly with missing details/recent updates and mentions of upcoming (in 2.7) changes;
“Highlights” added to the top of the each release with short (and subjective) list of the most visible changes;
Book styles updated a bit for easier reading;
More metainformation about each version added to its page (initial release date, current status, etc.).