Document include-only reStructuredText files

[jdillard]

Purpose

Explain that files matching source_suffix are treated as standalone documents, even when used with include and document best practices for creating reusable include-only content using a different suffix or exclude_patterns, preventing duplicate-label warnings.

The reactions and related reports indicate that this has been a recurring source of confusion for users, so thought it was worth making an improvement.

References

• Relates to sphinx-build reports duplicated label #9707
• Relates to .. include:: directive shouldn't care about filename extension #8515

AI Disclosure

AI was used to investigate the issue and draft the documentation change. I reviewed and validated the text.