docs: add BGP status reporting guidance for telemetry agent

[juan-malbeclabs]

Summary

• Adds optional --bgp-status-enable flag documentation to Step 4.5 of the Device Provisioning guide (contribute-provisioning.md), matching the existing VRF note style
• Adds a new BGP Status Reporting subsection to the Operations guide (contribute-operations.md) under Telemetry Agent Metrics, covering all four flags, their defaults, and the two new Prometheus metrics introduced in PR telemetry: submit BGP session status onchain per user doublezero#3487

Context

Implements RFC 19 (rfc19-user-bgp-status.md). The feature is opt-in — --bgp-status-enable must be explicitly passed to activate BGP session status submission onchain.

New flags documented:

Flag	Default	Description
--bgp-status-enable	false	Activates BGP status submitter
--bgp-status-interval	60s	Collection and submission frequency
--bgp-status-refresh-interval	6h	Periodic re-submission interval
--bgp-status-down-grace-period	30s	Minimum duration before reporting Down

Test Plan

• Run mkdocs serve and verify the new note renders in Contributors → Device Provisioning → Step 4.5
• Verify the new subsection renders in Contributors → Operations → Telemetry Agent Metrics → BGP Status Reporting
• Confirm admonition syntax and table formatting display correctly