Docs: add index for /customization/*

What does this MR do?

• Ports https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25298 to EE
• For https://gitlab.com/gitlab-org/gitlab-ce/issues/55234

Author's checklist

• Apply the correct labels and milestone
• Crosslink the document from the higher-level index
• Correctly apply the product badges and tiers
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails

Review checklist

• Technical writer's review (required)
• Merge this EE-MR first, CE-MR (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25298) afterwards