…o guides/api/securing-your-api (#48018)

Closes DOCS-1176

## Summary

This PR adds documentation-writing guidance for humans and agents, then
applies it to the “Securing your API” guide.

## Changes

- Add a documentation word list based on Google’s style guide and
existing MDX lint rules.
- Add a shared `docs-guides` Agent Skill with Cursor and Claude
integration.
- Expand contributing guidance for information types, procedures,
chunking, links, admonitions, grammar, and terminology.
- Restructure “Securing your API” into contextual and procedural
sections.
- Add section navigation, cross-references, transitions, and procedural
outcomes.
- Reduce repeated admonitions and improve scannability.

## Manual testing

1. Open `/docs/guides/api/securing-your-api` in Preview and compare to
Live.
https://docs-git-docs-restructure-api-supabase.vercel.app/docs/guides/api/securing-your-api
2. See that the content is improved and clear with no important context
removed.
3. See the Admonitions that are no longer marked as admonitions. See the
content still makes sense.
4. Review the diff of `CONTRIBUTING.md` and `WORD_LIST.md`.
5. See that you agree with the new rules and that they are clear.


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Documentation**
* Updated documentation-writing guidelines with clearer standards for
structure, formatting, components, diagrams, terminology, and
navigation.
* Added a comprehensive word and style reference for consistent
documentation language.
* Reworked the API security guide with clearer guidance on grants, RLS,
dedicated schemas, pre-request checks, rate limiting, and API keys.
* Added documentation authoring workflow guidance, including validation
and formatting steps.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->