docs: update documentation for v0.6.9

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Purpose

Keep documentation current with the v0.6.9 release.

Docs added/updated

• docs/content/development/environment-variables.mdx — documents WS_MAX_MESSAGE_SIZE, WS_IDLE_TIMEOUT, and WS_RATE_LIMIT for SDK connector hardening.
• docs/content/development/connector.mdx — updates connector WebSocket path to /connector/ws, adds connector hardening controls, and explains metrics-only registration behavior.
• docs/content/sdk/connector/index.mdx — adds a metrics-only connector workflow where project_id is optional for @metric-only scripts.

Codepaths covered

• sdk/src/rhesis/sdk/decorators/metric.py
• sdk/src/rhesis/sdk/clients/rhesis.py
• sdk/src/rhesis/sdk/connector/manager.py
• sdk/src/rhesis/sdk/connector/schemas.py
• apps/backend/src/rhesis/backend/app/routers/connector.py
• apps/backend/src/rhesis/backend/app/services/connector/schemas.py
• apps/backend/src/rhesis/backend/app/services/connector/handlers/registration.py
• apps/polyphemus/src/rhesis/polyphemus/routers/services.py

Knowledge gaps addressed

• Connector WebSocket hardening environment variables and defaults were not documented.
• Metrics-only connector operation (no project_id) was not documented in SDK connector docs.
• Development connector doc used an outdated WebSocket endpoint path.

Testing

• Confirmed all code examples match source behavior
• No unescaped curly braces in MDX prose
• New directories include _meta.tsx
• Internal links resolve to existing pages