generator: Replace GNOME wiki links with vala.dev

[sidt4]

• Updated links
• Removed vala for java programmers link.

[sidt4]

I'm not sure about the video links:

1. Vimeo (link 1) says video is unrated and requires creating a new vimeo account.
2. Some videos are really old (2011). Not sure if those are still relevant. Maybe we could reorder based on relevance. Some hint next to the link would also be helpful.

[tintou]

Having more up-to-date videos would be great, let's deal with it on another PR if that's necessary