docs: Terraform shadow link management (DOC-2022)

[Feediver1]

Resolves https://redpandadata.atlassian.net/browse/DOC-2022

Adds Terraform as a management option on the Configure Shadowing page, covering the redpanda_shadow_link resource shipped in Redpanda Terraform provider v2.0.0 (PR #343; current release v2.1.2).

Changes

• Create a shadow link: new Terraform tab in the cloud-gated tabs block (alongside Cloud UI / rpk / Control Plane API). Shows the redpanda_secret + redpanda_shadow_link pattern with TLS and SCRAM auth, the ${secrets.<name>} password reference (no plaintext in state), placeholder explanations in the page's existing style, and a NOTE covering the enable_shadow_linking prerequisite (settable via redpanda_cluster.cluster_configuration.custom_properties_json) and the allow_deletion = true destroy guard.
• Update an existing shadow link: new Terraform tab noting that terraform apply sends only changed fields (the provider uses a sparse FieldMask), and that name and cluster IDs are immutable (changes plan a replacement).

Both edits are inside existing ifdef::env-cloud[] regions of the single-sourced page, so they render only in the cloud build; the Self-Managed build is unchanged. The cloud-docs stub picks the content up automatically.

Review notes

• The HCL example is adapted from the provider's own examples/shadow_link/main.tf and trimmed to the minimal verified surface. Sync-option blocks are deliberately not shown inline; the tab links to the registry reference and the provider example for the full schema, to avoid drifting from generated docs.
• The provider example's ACL blocks are not reproduced because they grant fewer permissions than this page's "Replication service permissions" prerequisites require (missing describe_configs); the tab points at the prerequisites instead. Flagged to the provider team separately.
• The provider does no client-side check of enable_shadow_linking (verified in resource_shadowlink.go Create()), so the property is framed as a prerequisite, not a validated field.
• No Private Link claims are made; PL/PSC validation for shadow links was still in flight per the engineering epic's last update.
• Companion cloud-docs PR adds the shadow_link bullet to the Terraform provider resource list.

🤖 Generated with Claude Code

Preview pages

• Configure Shadowing (updated)

Note for reviewers: this preview renders the Self-Managed variant, which is intentionally unchanged (verified: zero Terraform mentions). The new Terraform tabs are inside ifdef::env-cloud[] regions and render only in the cloud build. Rendering was verified in a local cloud-docs build with the playbook pointed at this branch: both tabs appear in the create and update tab blocks, the HCL example and nested NOTE render cleanly, and no AsciiDoc directives leak.

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