Mutation filter: question mark

[chaoran-chen]

Show a question mark that provide additional information about the mutation filter syntax

[fengelniederhammer]

See https://github.com/GenSpectrum/dashboards/pull/149#discussion_r1568656962. As discussed:

• Show the ? button that we also have for the diagram. The button should show information on how to use the mutations filter field.
• Maybe also give examples of valid input in the placeholder text.