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

### 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

Start with the druid.apache.org/community/ page and the speaker guidelines at druidsummit.org/speaker-guidelines/, then review apache/druid-website-src/pull/505 for the existing website work. Done means the speaker guidance is available from the community page as a central reference and duplicate or contradictory resources are addressed.

Written by the indexing model from the issue text.