Remove OpenShift cluster sizing page

[johnwalicki]

Pull Request Template

Description

Please include a summary of the change and which issue is fixed. Please also include relevant motivation and context. List any dependencies that are required for this change.

Fixes #599

• Remove cluster_sizing.md
• Resolve the backup_recovery.md broken link
• Update hub/hub.md and hub/overview.md which references cluster_sizing.md

Type of change

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• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

Additional Context (Please include any Screenshots/gifs if relevant)

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Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have checked my code and corrected any misspellings
• I have tagged the reviewers in a comment below incase my pull request is ready for a review
• I have signed the commit message to agree to Developer Certificate of Origin (DCO) (to certify that you wrote or otherwise have the right to submit your contribution to the project.) by adding "--signoff" to my git commit command.