Clarify social media guidelines regarding boosting of non Kubernetes/CNCF events

Currently there isn't a clear definition of what should/shouldn't be considered an event that can be promoted. To ensure a neutral stance, the guidelines should be updated to state that only Kubernetes community or CNCF hosted events can quality for promotion.

Both the social media guidelines and issue template should be updated.

/sig contributor-experience
/area contributor-comms

Locate the social media guidelines and the issue template referenced in the issue, then read their current event-promotion wording. Update both to clearly limit promotion to Kubernetes community or CNCF-hosted events, and verify that the guidance is consistent in both places.

Written by the indexing model from the issue text.