feat: minor redesign

[mihai-sysbio]

In this PR, I aimed to refresh/improve the website in preparation for wider usage, by:

• including some calls to action on the landing page
• making the Sources page more user friendly
• streamlining the Documentation page
• simplyfing the navigation bar, including the Publication page

Some further to-dos are suggested below, either as further commits in this PR or to report as separate issues:

• updating the Gitter channel - the current one is for bio.tools
• the mermaid snippet in Documentation fails to be rendered
• the Applications page still used the old/tabular design

See some screenshots below:

Note: As soon as the other PR gets merged, this one will be only contain the redesign commit.