Document Submit Communities page

[zubeydecivelek]

https://github.com/CERNDocumentServer/docs-cds-rdm/blob/main/docs/communities/submit.md

Add documentation that explains:

• Metadata checks: What is checked when submitting to a community and how to fix issues.
• Multiple communities: How to submit the same record to several communities.
• Remove: How to remove a record from a community and when that is allowed.
• CERN Research: Explain the community and any special rules or workflow to submit.