CLOS-5756: document LVE per domain

[alutsiuk123]

The Isolates page covered the CageFS half of CloudLinux Isolates plus the user-side isolatectl limits; the administrator and observability half of LVE-per-domain was undocumented. Nothing in the repo mentioned list-domains, allow-domain-limits, enable-domain-limits, regenerate-domains, --with-domains, isolatectl stats, or keep_history_days_domain.

Isolates page:

• name the two layers (CageFS / LVE per domain) up front, and add an "LVE Per Domain" section as a sibling of "CageFS Per Domain";
• state the prerequisites as minimum package versions — lve-stats3 5.1.0-1 (CLOS-5756) and lve-utils 6.6.40-1 (CLOS-6820, isolatectl stats) — plus the CL8/CL9 kernel requirement;
• describe the container hierarchy, since the account's own LVE and its domain LVEs being siblings explains most of what the statistics show;
• document isolatectl stats beside the other isolatectl subcommands, including that an idle site is absent rather than zero, and that the period grammar reads 1m as one minute;
• record where the state lives (~/.lve/domains.json, /etc/container/lvd_ids/), which the existing "re-apply after editing the config" instruction never named.

Command line tools:

• lvectl: the six per-domain commands, and that cagefsctl already drives them under a control panel;
• lveinfo/cloudlinux-statistics: --with-domains and --domain, the three identity columns, and the domains[] shape;
• cloudlinux-top: --domains, called out against the pre-existing and unrelated -d/--domain filter;
• lvechart: --domain.

Components:

• keep_history_days_domain, and that keep_history_days caps it;
• the domain_faults notification variable, and that the user email subtracts a site's faults from the account line while every other surface rolls them up.

Verified with yarn docs:build (49 pages, no errors).

Worked-On: cl-aiworkspaces