docs: document mark and scope event sanitizers [6/6]

[willkill07]

Overview

Documents the RELAY-409 event sanitizer and PII redaction behavior in one independent, docs-only PR so the writing review is not repeated across the implementation stack.

• I confirm this contribution is my own work, or I have the right to submit it under this project's license.
• I searched existing issues and open pull requests, and this does not duplicate existing work.

Details

• Adds a dedicated event sanitizer reference with callback semantics, ordering, registration lifetimes, cross-language APIs, and dynamic-plugin protocol values.
• Updates middleware, event, scope/mark, plugin authoring, and pending-mark guidance.
• Documents PII mark = true defaults, opt-out behavior, broader input/output scope coverage, and exporter privacy guarantees.
• Updates Python, Node.js, and experimental Go package READMEs plus relevant agent skills.
• Is based directly on main and contains no implementation commits.
• Should be finalized after PRs feat(sanitizers)!: add mark and scope event sanitizer registries [1/6] #371–feat: expose event sanitizers through C and Go [5/6] #375 establish the documented APIs.
• Breaking changes: none.

Validation: just docs (zero errors; remote redirect comparison skipped after FDR returned 403) and uv run pre-commit run --all-files.

Where should the reviewer start?

Start with docs/reference/event-sanitizers.mdx, then review docs/pii-redaction-plugin/configuration.mdx for the privacy contract changes.

Related Issues: (use one of the action keywords Closes / Fixes / Resolves / Relates to)

• Relates to: RELAY-409

[github-actions]

Fern docs preview: https://nvidia-preview-pull-request-376.docs.buildwithfern.com/nemo/relay (​https://nvidia-preview-pull-request-376.docs.buildwithfern.com/nemo/relay​)