Add documentation for indexing batch size configuration

[kjac]

📋 Description

umbraco/Umbraco-CMS#20543 introduces a configurable batch size for large indexing operations. This is the documentation part of that configuration 😄

📎 Related Issues (if applicable)

None

✅ Contributor Checklist

I've followed the Umbraco Documentation Style Guide and can confirm that:

• Code blocks are correctly formatted.
• Sentences are short and clear (preferably under 25 words).
• Passive voice and first-person language (“we”, “I”) are avoided.
• Relevant pages are linked.
• All links work and point to the correct resources.
• Screenshots or diagrams are included if useful.
• Any code examples or instructions have been tested.
• Typos, broken links, and broken images are fixed.

Product & Version (if relevant)

17.1

Deadline (if relevant)

Not before Jan 6th, 2026 (that's the launch of 17.1).

📚 Helpful Resources

• 🧾 Umbraco Contribution Guidelines
• ✍️ Umbraco Documentation Style Guide

[eshanrnh]

Thanks for the PR, @kjac 🙌 The PR looks good 💪