feat: support index.md as section file

[klocke-io]

Summary

• Adds proposal to extend docforge's hardcoded _index.md section file detection to also recognize index.md
• VitePress and other modern SSGs use index.md as section index — currently this bypasses the sectionFile special case in resolveManifestLinks, breaking overlay manifests silently
• The docsy plugin already handles both names (plugin.go:22), confirming this was a known gap

Motivation

When downstream repos migrate from Hugo to VitePress and rename _index.md → index.md, docforge no longer auto-generates the section index from frontmatter. Instead it treats it as a normal file, resolves its source, and fetches real content — overwritingl section metadata.

Proposal

Minimal approach: add an isSectionFile(name string) bool helper that matches both _index.md and index.md, update the 5 affected call sites. Full details in the proposal document.

Test plan

• Review proposal at openspec/changes/support-index-md-as-section-file/proposal.md
• Discuss approach and approve before implementation

[gardener-prow]

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