feat: add document icon for doc.md and docs.md files

[tupe12334]

Description

Maps files named doc.md / docs.md, and the *.doc.md / *.docs.md extensions, to the existing document icon.

Follows the same approach as #3483 (benchmark markdown icon): a markdown-blue (#42a5f5) document glyph for documentation-flavored markdown files. Reuses the existing document icon rather than adding a new SVG. The more-specific doc.md/docs.md extension match takes precedence over the generic .md markdown mapping (same mechanism as bench.md).

• Registered in fileIcons.ts:
  • file names: doc.md, docs.md
  • file extensions: *.doc.md, *.docs.md

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