Add environment variables table to Configuration Overview

[nginx-seanmoloney]

Proposed changes

This PR adds a new NGINX Agent configuration options section to the configuration-overview.md documentation. It provides a clear table listing all environment variables, their descriptions, and default values to help users configure the agent correctly.

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[ADubhlaoich]

Generally LGTM.

@JTorreG I feel the table information already is or could be an include for re-use.