ESC docs: fix and upgrade rotator provider pages

[jkodroff]

Part of #19378 (Refresh ESC Docs); related to #19690 (ESC: Revise providers pages).

Scope: the content/docs/esc/providers/rotators/** tree.

These items surfaced during the audit/work for #19463 but were intentionally split out so that PR could stay focused on the secrets/* pages. This issue tracks the rotator-specific bugs and the rotator template-upgrade work.

A. Concrete bugs found

• passphrase.md — intro sentence is grammatically broken ("The passphrase rotator enables you to any user defined key in your ESC environment generating memorable passphrases"); should read "enables you to rotate any user-defined key … by generating memorable passphrases". The YAML example is also invalid — it's missing the parent map key under values: (jumps straight to fn::rotate::passphrase:); compare password.md's mypassword:. Plus a stray blank line inside the code fence and "user defined" → "user-defined".
• postgres.md — empty ## Setup stub heading between the State example and ## Inputs; and the example host: uses a -mysql RDS hostname (rotator-example-mysql.cluster-…) in a postgres example.
• mysql.md — empty ## Setup stub heading.
• snowflake-user.md — Steps 5–6 are ## while Steps 1–4 are ### under the same "Configuring Snowflake for Key Rotation" parent; normalize the heading levels.
• external.md — the JWKS URL in the sample (https://api.pulumi.com/.well-known/jwks.json) returns 404. It should be https://api.pulumi.com/oidc/.well-known/jwks, which is what every other ESC page uses (verified live: /oidc/.well-known/jwks → 200, the .json form → 404).

B. Template gaps across the rotator pages

Per the canonical provider-page structure in #19463:

• No Troubleshooting section on any of the 8 rotator pages.
• No Related cross-links section on any of the 8 rotator pages.
• No esc open sample output: postgres, mysql, password, passphrase.
• Static-credentials / alternate-login example missing despite schema support: aws-iam, azure-app-secret, snowflake-user are OIDC-only in their examples.
• password.md and passphrase.md lack State/Outputs reference tables entirely (every other rotator has them).
• Schema-reference scannability/consistency — align section headers so they're scannable; consider driving them from a shortcode or data source (ties to ESC: Revise providers pages #19690's stated goal).

Note: Prerequisites/Troubleshooting content should be SME-verified (exact permissions, error strings) before publishing.