banner for UGM

[orbeckst]

• basic MDA banner for user group meeting
• requested in https://github.com/MDAnalysis/MDAnalysis.github.io/pull/278#discussion_r1197607901

[orbeckst]

@jennaswa @micaela-matta please have a look and make suggestions for changes.

[orbeckst]

Here you go:

[orbeckst]

@micaela-matta @jennaswa please merge if you're happy with it, otherwise do a request changes review and tell me what you need. Thanks!