gen-docs-reference: preserve hand-written feature-gated nodes section

[staging-devin-ai-integration]

Context

The docs generator (apps/skit/src/bin/gen-docs-reference.rs) regenerates docs/src/content/docs/reference/nodes/index.md from the compiled node registry. Feature-gated nodes (openh264, mp4, rtmp, etc.) are not compiled into the default binary, so the generator can't discover them.

A hand-written "Feature-gated nodes" section at the bottom of the index lists these nodes. If the generator is re-run, it will overwrite this section.

Proposed fix

Update render_nodes_index() in gen-docs-reference.rs to either:

1. Append a static feature-gated section after the generated content, or
2. Read the existing file and preserve content below a marker comment (e.g. <!-- manual-content-below -->)

Found during the v0.5 docs review (#407).