Add KOSS Communication Platform Guides & Mention The Archive

There is no single guide about which communication platforms KOSS uses. The [onboarding](https://github.com/kossiitkgp/docs/blob/master/community/onboarding-offboarding.md) doc mentions the platforms.

Since we use a lot of platforms now - Slack, WhatsApp, Google Group, we should have a separate document listing all the platforms and guidelines for how to use each (eg, Slack for official communication, Google Group for something that needs to be broadcast to everyone, etc.). Guidelines for setting up meetings can also be included in this as Google Meet is also used often.

Lastly, the slack archive: https://opsa.kossiitkgp.org, should also be mentioned so it is documented and maintained in the future.

Start with community/onboarding-offboarding.md to review the existing platform references. Create a separate communication-platform guide covering Slack, WhatsApp, Google Group, Google Meet, and the stated usage guidelines, and mention the Slack archive at opsa.kossiitkgp.org. Done means the platforms, meeting guidance, and archive are documented in one maintainable guide.

Written by the indexing model from the issue text.