Update website to reflect WG merged into OpenGitOps project

[scottrigby]

The WG has merged into the OpenGitOps project. See open-gitops/project#172

• Write blog announcement (I'm happy to collab on this with anyone interested to contribute 🙂)
• Add banner with this announcement (see announcement I added to the top of the meeting notes doc)
• Change descriptions throughout the website to reflect this change (search for "group", "wg". Historic references should be kept but any current info should be updated)
• Update getting involved page to point to the right meeting calendar info #132