Restore v9 TOC menu sub-items

[hsubox76]

Revert part of #5333 so that we go back to generating TOC sub-items for each product.

[changeset-bot]

⚠️ No Changeset found

Latest commit: 93f1e7c

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[github-actions]

Changeset File Check ⚠️

• Changeset formatting error in following file:

  Some packages have been changed but no changesets were found. Run `changeset add` to resolve this error.
  If this change doesn't need a release, run `changeset add --empty`.
  

[google-oss-bot]

Size Report ¹

Affected Products

No changes between base commit (19e37af) and merge commit (716a478).

Test Logs

• Base (19e37af): https://github.com/firebase/firebase-js-sdk/actions/runs/2417126234
• Merge (716a478): https://github.com/firebase/firebase-js-sdk/actions/runs/2417171161

1. https://storage.googleapis.com/firebase-sdk-metric-reports/NvSrvsUlQm.html

[google-oss-bot]

Size Analysis Report ¹

Affected Products

No changes between base commit (19e37af) and merge commit (716a478).

Test Logs

• Base (19e37af): https://github.com/firebase/firebase-js-sdk/actions/runs/2417126234
• Merge (716a478): https://github.com/firebase/firebase-js-sdk/actions/runs/2417171161

1. https://storage.googleapis.com/firebase-sdk-metric-reports/H443WdTuuL.html

[dwyfrequency]

LGTM