[codex] docs: add pipeline extension recipe

[ketpatil77]

Description

Headroom already supports headroom.pipeline_extension, but the request-normalization pattern was not documented. That leaves users guessing how to fix upstream quirks such as content: null tool-call payloads.

Changes Made

• Added a Pipeline Extensions section to configuration.mdx.
• Documented the PRE_SEND hook as the right place for request cleanup.
• Included a minimal NormalizeNullContent example and entry-point registration.
• Mentioned x-headroom-base-url as the per-request routing override.

Testing

• Reviewed the docs diff for API names and placement.
• Confirmed the example uses the public headroom.pipeline contract.

Impact

Users can implement provider-specific request cleanup without forking Headroom, and can pair it with per-request upstream routing when needed.

[github-actions]

PR governance

This PR does not yet satisfy the required template fields:

• Missing required section Type of Change.
• Missing required section Real Behavior Proof.
• Missing required section Review Readiness.
• Paste real command output or artifact links in Testing → Test Output.

Please update the PR body, or move the PR back to draft while it is still in progress.