Move Issuer / ClusterIssuer and Certificate resource content to a sub-folder of configuration/

[wallrj]

Preview: https://deploy-preview-1075--cert-manager-website.netlify.app/

Part of the https://github.com/cert-manager/website/pull/1048 where we presented an improved overall navigation structure for the site.

• Moved configuration/* > configuration/issuer-and-clusterissuer-resources/*
• Moved usage/certificate > configuration/certificate-resources
• Updated all the affected links within the site.
• Added redirects for the benefit of people linking to our site
• Improved the page descriptions and opening paragraphs of each of the Issuer type pages and the certificate resource page
• Added a list of all the issuer types to the configuration/issuer-and-clusterissuer-resources/ homepage, because the side menu is not always expanded when you visit that page.

/cc @maelvls @mehak151 (and @jsoref ...if you have time)

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[jetstack-bot]

@wallrj: PR needs rebase.

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[inteon]

This has been solved in an alternative way.

TODO: recover the changes made in the following files & apply them to the new website structure (if possible)

• content/docs/configuration/README.md
• content/docs/configuration/certificate-resources.md
• content/docs/configuration/issuer-and-clusterissuer-resources/README.md