202501 Community event guidelines

[petermarshallio]

This PR contains a draft set of organizer and speaker guidelines for community events, particularly influenced by the ASF's own policies and guidance, the guidance of other Apache communities such as the Apache Kafka community, and guidelines recently used for Druid Summit 2024, hosted by Imply.

I propose that this be included on the main Apache Druid website to provide:

• An opinionated view on what a Druid community event is, and the standards to be met.
• Practical information for organizers on how to partner with PMC members.
• Clear guidance (and encouragement!) for would-be speakers at community events.
• Concrete expectations for speakers or organizers who come from a more commercial environment.

[petermarshallio]

Discussed with members of the project. After discussion, consensus is that this duplicates what is already out there (e.g. ASF) and isn't consistent with approaches in other projects.