docs: host Cachet separately, plus v3.x feature corrections and additions

[jbrooksuk]

Summary

Adds a hosting best-practice note to the v3.x introduction, then reconciles the v3.x documentation with the current behaviour in cachethq/core — correcting several inaccurate pages and documenting features that were missing.

Hosting guidance

• Introduction — adds a <Warning> recommending that Cachet be hosted on infrastructure independent of the application/service it reports on, so the status page stays online during an outage.

Corrections (docs disagreed with core)

• Components — component statuses said "five"; there are six. Adds the missing Under Maintenance (6) status.
• Beacon — telemetry is enabled by default (config/cachet.php: env('CACHET_BEACON', true)). Docs previously said it was off by default. Now documents opting out via CACHET_BEACON=false.
• Cachet settings — removes the stale "metrics are not fully supported" warning; metrics are implemented and fully documented.
• Dashboard — replaces "Cachet currently does not support user management" (which contradicted the Users guide) with a pointer to the Users guide.

New & expanded coverage

• Environment variables — new configuration/environment.mdx reference page covering the config/cachet.php variables, grouped by area, and added to the Configuration navigation.
• Components — group visibility, collapse behaviour (expanded / collapsed / collapsed-unless-incident), and ordering strategies.
• Incidents — stickied (pinned) incidents.
• Schedules — affected components (with the status they display) and subscriber notifications.
• Metrics — "show when empty" option and metric reordering.
• Cachet settings — language/localization, "recent incidents only", and dynamic favicon.
• API keys — enumerates the resource abilities (manage / delete per resource).
• API filtering — documents include (related resources) and filter[meta][key] (metadata filtering).

Notes for reviewers

• Beacon default: the docs now match core, where telemetry defaults on. If the distributed application ships an .env.example that sets CACHET_BEACON=false, the effective end-user default would differ — worth confirming.
• The existing Timezone section on the Cachet settings page was left in place and cross-referenced from the new Language section, rather than relocated. These settings actually live on the "Manage Localization" page, so a follow-up could regroup them.

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