Render fragment content organization and cross-links

[guardrex]

Addresses #24615

I started this effort with a short render fragment section, but the final content layout seems better:

• Explain the child content scenario in place with an updated heading. Cross-link other articles to this for child content examples. Cross-link the existing section to those as examples of where/how child content is set. Include a NOTE about how built-in components use the same convention.
• Add a short section for reusable rendering logic. Leave that content back in the Performance topic ... link to it from here.