[DOCS] Add details about regenerating CLI schema

[lcawl]

While working on #3513 I noticed that the new command options were not automatically added to the CLI schema and there weren't obvious instructions for doing so.

This PR makes the steps more explicit.

AI summary

1. CONTRIBUTING.md
   Added CLI reference maintenance after the watch section:

   • Regen command: dotnet run --project src/tooling/docs-builder -- __schema > docs/cli-schema.json
   • CI drift check note (version field ignored)
   • Pointer to supplemental docs in docs/cli/

2. docs/cli/cli-reference-how-to.md
   Added a Maintaining docs-builder itself note in step 1 with the same regen command and a link to CONTRIBUTING.

3. AGENTS.md and CLAUDE.md
   Added a CLI changes bullet under Essential Commands with the regen command and CONTRIBUTING link. CLAUDE.md already matched (likely kept in sync with AGENTS.md).

Generative AI disclosure

1. Did you use a generative AI (GenAI) tool to assist in creating this contribution?

• Yes
• No

2. If you answered "Yes" to the previous question, please specify the tool(s) and model(s) used (e.g., Google Gemini, OpenAI ChatGPT-4, etc.).

Tool(s) and model(s) used: composer-2.5-fast

[Mpdreamz]

Still need to add a PR check for this to ensure we generate this as part of the PR. Did that for elastic/cli but not our own :)

[coderabbitai]

📝 Walkthrough

Walkthrough

Three documentation files are updated to describe the CLI schema regeneration workflow. CONTRIBUTING.md gains a new "CLI reference maintenance" subsection explaining that contributors must run dotnet run ... -- __schema > docs/cli-schema.json after changing CLI options in src/tooling/docs-builder/Commands/, commit the result, and that CI enforces schema drift (excluding the version field). AGENTS.md adds a parallel "CLI changes" instruction block with the same command and a reference to CONTRIBUTING.md. docs/cli/cli-reference-how-to.md inserts a note callout in the "Create a schema file" step pointing to the same regeneration command and CONTRIBUTING.md.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and specifically describes the main change—adding documentation details about the CLI schema regeneration process.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description check	✅ Passed	The pull request description clearly relates to the changeset, explaining the motivation (missing instructions for CLI schema regeneration) and summarizing the updates across multiple documentation files.

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✨ Finishing Touches

✨ Simplify code

• Create PR with simplified code
• Commit simplified code in branch cli-docs

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