Add list of deRSE confs to Events > deRSE conferences

[sdruskat]

Submenu item to be added as well.

[nuest]

@sdruskat What about the conference website for last year - it will not receive the new design, right?

[sdruskat]

No, this should probably be just a static version of what's there now. Could try and move that to a separate repo and use forwarding/path logic on GitHub Pages to achieve something useful (away from this repo). The easiest way would be to do it hte same way that the deRSE20 site works now. Or you can introduce /conferences/ or similar, just like for chapters? I'd be wary of having to slog all that binary stuff on the deRSE19 site through every build though ;).