[DOCS] Add reference info for docset.yml

[lcawl]

Summary

Add comprehensive documentation for the docset.yml file format in the content-set configuration section, specifying which fields are required vs optional and their purposes.

Context

The docset.yml file (or _docset.yml) configures a content set's structure and metadata. Currently, documentation is scattered across navigation.md, building-blocks/documentation-set.md, and other pages. There is no single reference that lists all supported fields and their required/optional status. The products field, for example, is optional (defaults to empty) but this is not documented.

I identified this gap when I was reviewing https://docs-v3-preview.elastic.dev/elastic/docs-builder/tree/main/syntax/changelog and trying to figure out what product ID defaults that were "auto from docset" were coming from.

Source of truth

The schema is defined in:

• TableOfContentsFile.cs – base fields: project, toc, suppress
• DocumentationSetFile.cs – extended fields: max_toc_depth, dev_docs, cross_links, exclude, extensions, subs, display_name, description, icon, registry, features, api, products, codex
• ConfigurationFile.cs – how fields are processed (e.g., products only applied when Count > 0)

Implementation

• Add docs/configure/content-set/docset-format.md as the canonical reference for the docset.yml file format.
• Update docs/configure/content-set/index.md to add a link to the new docset format reference in the "Learn more" section.
• Update docs/building-blocks/documentation-set.md and docs/configure/content-set/navigation.md to reference the new docset-format.md for the full field list, avoiding duplication.

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-1.5

[github-actions]

🔍 Preview links for changed docs

• docs/configure/content-set/docset-format.md
• docs/building-blocks/documentation-set.md
• docs/configure/content-set/index.md
• docs/configure/content-set/navigation.md

[lcawl]

Suggested change

	# docset.yml file format
	# docset.yml