docs: clarify sendFile dotfiles migration

[jhcpeixoto]

Closes #1868

Summary

• clarify that the Express 5 res.sendFile() dotfiles option applies to hidden directories in the path, not just hidden target files
• add an example matching the hidden mount directory case from v5 sendFile fails with NotFoundError express#6450

Verification

• npm run check
• npm run build
• git diff --check

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[jhcpeixoto]

I checked the failing linkChecker job. This PR adds text only and no new links. The failures are existing site-wide links such as https://slack-invite.openjsf.org/ returning 403, LinkedIn profile links, old Node API anchors, and translated-page local paths; they are not introduced by this change.