Improve links in contribution description

Multiple things:

• the topmost README.md should link to doc.zih.tu-dresden.de/docs/contrib/howto_contribute.md (instead doc.zih.tu-dresden.de/README.md)
• howto_contribute.md should link to contribute_browser.md (instead providing explanation itself)
• contribute_browser.md should link to content_rules.md
• contribute_container.md should link to content_rules.md
• remove contribution things from doc.zih.tu-dresden.de/README.md (or remove it altogether)