Update information for Marvin Beckers

[embik]

Hey folks! 👋🏻

I would like to add some more information to my profile, so here is a PR that updates things and puts them a bit more into standard (I think the personal website in the bio is kind of unusual, and there's a field for it anyway).

I'm a little confused about slack vs slack_id. Many entries have the slack field, but slack_id seems to be the right one according to schema.json. If this correction is not accurate, please let me know and I will drop it from the commit.

[cjyabraham]

Thanks for calling this out. I've replaced all "slack" attributes with the correct "slack_id". This caused conflicts in this PR, so I made the changes you wanted manually. If I made an error or you want something else changed, please open a new PR.