Add Speaker's Guide to druid.apache.org as event guidance

[hevansDev]

Description

Add the speaker's guide from https://druidsummit.org/speaker-guidelines/ to https://druid.apache.org/community/ as per request from @petermarshallio to create a central point of reference for these guidelines / rules similar to https://developer.confluent.io/community/rules-of-engagement/.

See https://github.com/apache/druid-website-src/pull/505

Motivation

• Clarify rules on using Apache Druid brand name in events
• Provide a useful reference for speaker guidelines
• Eliminate duplicate, contradictory resources