docs: correct v0.116 Linux package guidance

[GuanzhouSong]

Summary

Correct the website and package guide for the redesigned Linux packaging system introduced in DocumentDB v0.116-0.

The paved path remains intentionally short for a first-time user on a clean host:

1. Install the full-stack package.
2. Run the deterministic fresh-instance setup command.
3. Connect with mongosh.

Detailed brownfield, unattended, TLS, upgrade, reset, and no-systemd workflows live in the linked operations guide.

Changes

• Pin fresh setup to the selected PostgreSQL major and force a new private instance, avoiding accidental adoption or silent selection of another installed major.
• Split EL9 prerequisites between Rocky Linux, AlmaLinux, and CentOS Stream (crb) and registered RHEL (subscription-manager / CodeReady Builder).
• Separate safe greenfield reset from brownfield restore so setup state is not destroyed before cleanup determines which data belongs to the package.
• Document the complete TLS override, including disabling automatic certificate generation and granting the gateway read access to the private key.
• Replace unsafe upgrade guidance with the v0.116 compatibility boundary and the required full-stack, PostgreSQL-restart, extension-update, and gateway-restart sequence.
• Add the supported brownfield adoption workflow, including the possible operator-managed PostgreSQL restart and second setup run.
• Distinguish scoped systemd restore from no-systemd cleanup, where v0.116 requires an unscoped restore to stop nohup gateways.
• Move the complete unattended command into the advanced guide and link to it from the Package Finder and quick start.
• Stop inferring one package's version suffix from another package in the advanced version-pinning panel.

E2E findings and user impact

Concern	Finding	User impact
Setup selection	Fresh PG17-only and PG18-only installs work, but multi-major or pre-existing-cluster hosts can prompt or silently choose PG18.	Conditional for new users; deterministic flags remove the ambiguity without complicating the paved path.
Restore before reset	Restore removes ownership and path state needed by reset.	Advanced-only, but potentially destructive or leaves custom package-managed data running.
Upgrade instructions	An extension-only v0.114 upgrade does not install the v0.116 stack, and updating extensions before restarting PostgreSQL can leave a partial catalog upgrade.	Severe for existing installs; not part of a first-time installation.
TLS replacement	Cert/key paths conflict with the default auto-generation setting, and an unreadable key prevents gateway startup.	Affects users replacing the development certificate for network exposure.
RHEL prerequisites	Rocky Linux 9 resolves through CRB; registered RHEL has no crb repository ID and requires CodeReady Builder through subscription-manager.	Blocks registered RHEL users before DocumentDB installation.
Brownfield adoption	The quick start linked to operations guidance that did not contain the supported --target-postgres-instance workflow.	Existing-cluster users reached a dead end; the advanced guide now contains the complete workflow.
No-systemd detach	Scoped restore cannot attribute and stop a nohup gateway in v0.116.	A brownfield endpoint could remain live after an apparent detach; the guide now requires the safe unscoped flow and port verification.

Validation

• Fresh Ubuntu 24.04 PostgreSQL 17 and 18 installs reached a working mongosh connection.
• Fresh Rocky Linux 9 installation reached a working mongosh connection.
• Reset, upgrade, TLS, registered-RHEL prerequisite, and no-systemd restore behavior were reproduced against v0.116-0.
• Package command and article regression tests: 49 passed.
• Full unit suite: 76 passed.
• ESLint passed.
• Next.js production build and local HTTP checks passed for /packages/, /docs/getting-started/packages/, and /docs/linux-packages/.
• GitHub Actions run 33525043424 passed YAML validation, the full site build, and package-repository dependency resolution for both PostgreSQL majors.