Create front page

[m-mohr]

Depends on #57, maybe also on #3.

Once we have more content on the Hub, e.g. process exchange (#57), UDF exchange (#3) or OIDC (#4), we should create a front page to give a quick overview. We should look how other similar pages do it, but it could list most popular items or recently updated items for each section. It should guide through the page, give an introduction and also have big buttons (see openeo.org's "Why?" and "Get Started!" for example) to guide to the subsections (as the navigation is pretty small).