[DOCS] Establish and publish flagd communication policy

[cupofcat]

Define official channels (GitHub Releases, Release Notes) and supplementary channels (Slack, website blog)

[toddbaert]

Regarding releases, we already have a CHANGELOG and release-please, which automates things related to new features, etc.

We also have the flagd slack channel for other things, and a flagd Twitter/X.

Going forward, the only additional thing I can think we might consider in the future is enabling the blog functionality on https://flagd.dev/

I would motion to close this otherwise.

[cupofcat]

This issues is also about documenting clearly which kinds of changes are published in what channels so the users know the "contract" about discovering things like breaking changes, deprecations, CVEs, etc.