Fix styling issues in use-jekyll branch to get it ready for deployment

[TabrisArg]

What's the problem

Several CSS related issues must be fixed before we can merge this cranch with master

Requirements

• fix the white margins apprearing in the "become a developer", "workshops" and"blogs" pages
• center alignment in the "contact us" button
• center the workshops' titles appearing in the workshops page
• fix the margins appearing in the footer in some pages (Workshops)
• add titles to the pages