Some more vala documentation, listed in gnome docs

[nick87720z]

Looking to https://wiki.gnome.org/Projects/Vala/Documentation, i found much more links to vala tutorials. Some of them would be great to appear on start screen of this app.

For instances:

• suprisingly, there is "Getting Started" book from Elementary OS team, including some articles, already highlighted on their news channel at medium.com (i did not know, that such doc exists).
• Official Vala Reference from vala project site would be handy as well. May be more - i don't know all, this is just what i got into head, and I'm not sure about even proposing to add entire contents from above url.