docs: Add Meeting Calendar information for Interest and Working Groups - #1608

Addresses #1401

Not all groups have been set up yet, however the site is now set up at the official domain so I think we can get these docs updated and make an announcement to groups to start using it.

Motivation and Context

Currently, there's no centralized way for community members to discover when Working Group and Interest Group meetings are happening. This makes it difficult for contributors to participate in the groups they're interested in, reducing engagement and making the collaborative process less accessible.

This PR introduces documentation for a public community calendar at https://events.modelcontextprotocol.io/ that will provide a single source of truth for all WG and IG meetings, making it easy to discover and participate in community discussions.

How Has This Been Tested?

• Calendar site (mcp.meetable.org) set up and ready to publish meeting schedules (thank you @aaronpk !)
• NOTE: CNAME still needs to be configured for events.modelcontextprotocol.io

Breaking Changes

None.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally (N/A for docs-only change)
• I have added appropriate error handling (N/A for docs-only change)
• I have added or updated documentation as needed

Additional context

This change adds:

1. A new "Meeting Calendar" section explaining the public calendar at events.modelcontextprotocol.io
2. Updated expectations for both Interest Groups and Working Groups to include publishing meeting schedules to the calendar
3. Clear responsibility assignment to Facilitators for maintaining calendar entries