Add guidelines for breadcrumbs

[sboldyreva]

If we implement breadcrumbs as additional navigation #551, it should be documented and added to the wiki for the future new pages. There's a Contribute to Documentation page that contains most of our policies/templates we try to stick to. It sounds as a right place to add about breadcrumbs

[redadmin-k]

Remove outdated IRC/Mattermost and update platform list (Fixes #563)

This PR updates the community platforms page to reflect the current communication tools used by the AlmaLinux community.

• Removes references to IRC and Mattermost
• Adds Matrix, Discourse, GitHub Discussions, Reddit, and Mailing Lists
• Aligns with modern collaboration practices

Fixes #563