docs: clarify how reference documentation is generated and updated

[phoekerson]

Fixes #9590

Discussion

As discussed in #9590, the "Updating Documentation" section of CONTRIBUTING.md
described the legacy Typedoc/compat process and conflicted with the
"Generating Documentation HTML Files" section below it. @hsubox76 invited
proposed improvements in the issue discussion.

This PR rewrites the section to explain:

• that reference docs are generated from TSDoc comments in each package's
  source code (with docs-devsite/ai.aimodel.md / AIModel as an example,
  as raised in the issue)
• that files in docs-devsite/ are generated and must not be edited directly
• the contributor workflow: edit the TSDoc comment, run yarn docgen:all,
  commit both the source change and the regenerated files
• that the Typedoc/compat process only applies to the legacy compat API
  (also fixes a broken toc.yaml path that pointed to a directory that no
  longer exists)

Testing

Documentation-only change (CONTRIBUTING.md), no code affected. No changeset
needed per the contribution guidelines.

API Changes

None.

[changeset-bot]

⚠️ No Changeset found

Latest commit: 173cd2f

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[gemini-code-assist]

Code Review

This pull request updates the CONTRIBUTING.md file to reflect the new documentation generation process using TSDoc, api-extractor, and api-documenter. The review feedback suggests minor improvements to the documentation, including using a relative link for the api-documenter path and replacing the placeholder <product> with <package> for consistency.

[gemini-code-assist]

Using a relative link for api-documenter is more robust and consistent with other links in this document, as it avoids hardcoding the main branch URL of the repository.

Suggested change

	The markdown files in `docs-devsite/` are generated by
	[api-extractor](https://api-extractor.com/) and a customized version of
	[api-documenter](https://github.com/firebase/firebase-js-sdk/tree/main/repo-scripts/api-documenter),
	The markdown files in `docs-devsite/` are generated by
	[api-extractor](https://api-extractor.com/) and a customized version of
	[api-documenter](repo-scripts/api-documenter),

[gemini-code-assist]

Using <package> instead of <product> is more consistent with the terminology used throughout this section (e.g., "package's source code").

Suggested change

	1. Edit the TSDoc comment on the relevant class, function, interface, or property in the
	package's source code (`packages/<product>/src/...`).
	1. Edit the TSDoc comment on the relevant class, function, interface, or property in the
	package's source code (`packages/<package>/src/...`).