Update website footer

[cesarhernandezgt]

The current footer has references to jekyll github and jekyllrb twitter accounts:

In terms of footer content (without focusing yet on design), my suggestion is to have at least:

1. Existing JUG mailing list https://jugs.groups.io/g/jug-leaders [This is already part of the footer]
2. Link to jugleaders slack https://jugleaders.slack.com/
3. Twitter: https://twitter.com/WWJUGs
4. Contribute! link pointing to website repo readme file: https://github.com/World-Wide-JUGs/GlobalWWJugs