docs: align Isolates commands, prerequisites and panel integration

[NikolayVoina]

The Isolates instructions mixed deprecated CageFS options with current commands and tied normal procedures to one package build. Use the current commands consistently and describe behavior without incidental audited-release qualifiers or release-channel labels. Feature minimums remain documented.

• Clarify partial success, document-root restrictions and regeneration without usernames.
• Make per-domain prerequisites actionable: inspect the installed lvectl command's help, check marker existence, and distinguish these checks from kernel and panel support.
• Preserve minimum package versions and OS support limits. State that Isolates requires CloudLinux OS 8 or later, retain the per-domain LVE requirements, and give concrete troubleshooting steps.
• Correct panel/native-PHP setup guidance and document the custom-panel domain deletion hook, including its cleanup conditions.

Validation: the official frozen-lockfile Yarn install and VuePress docs build pass with Node 22. All four generated pages match the edited sources; new anchors and command examples were checked. Static checks report no new failures; 70 existing findings also occur at the pinned master baseline. Changed commands and conditions were compared with published package contents; no product was installed or OS/panel command executed.

This is a manual revision, updated to address the seven documentation-review comments on Isolates prerequisites and availability. The earlier automated scores apply to the preceding revision; the paid audit was not rerun. Product-engineer and documentation-owner review are still required. The PR remains Draft.

[alutsiuk123]

LGTM