Backport of Add reading guide for designing Vault clusters into release/1.15.x

[hc-github-team-secure-vault-core]

Backport

This PR is auto-generated from #26947 to be assessed for backporting due to the inclusion of the label backport/1.15.x.

:rotating_light:

Warning automatic cherry-pick of commits failed. If the first commit failed, you will see a blank no-op commit below. If at least one commit succeeded, you will see the cherry-picked commits up to, not including, the commit where the merge conflict occurred.

The person who merged in the original PR is: @schavis This person should manually cherry-pick the original PR into a new backport PR, and close this one when the manual backport PR is merged in.

merge conflict error: POST https://api.github.com/repos/hashicorp/vault/merges: 409 Merge conflict []

The below text is copied from the body of the original PR.

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Preview link: https://vault-9mygetkrz-hashicorp.vercel.app/vault/docs/enterprise/cluster-design

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Overview of commits

• b38fdef0126df4754ab1a1fdc4b883e34a43fb26

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CI Results: All Go tests succeeded! :white_check_mark:

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Build Results: All builds succeeded! :white_check_mark:

[schavis]

Closing as the information is mostly relevant to the current Vault version and may link to pages not available in older doc versions