Quick Start Guide, css changes, footer with logos and new index page

[lena-hinrichsen]

• the footer css changes are a workaround at the moment. We need it to be never overlapping with the toc but also not making the toc smaller by the padding (instead there should be whitespace depending on whether there is a toc or not). => @paulpestov can you have a look?

• added logos of partners in the footer

• new index page (ger version has to be added)

• Quick Start

• includes https://github.com/OCR-D/ocrd-website/issues/317 (remove docker compose reference)