[Feature]: surface root-level markdown in Fern docs without duplicating sources

[pdmack]

Prerequisites

• I searched existing issues

Feature Summary

Fern navigation currently cannot link to root-level markdown files (SECURITY.md, RELEASE_NOTES_v1.0.0.md, etc.) or section directories (runbooks/, configuration/).
Links from docs/OVERVIEW.md to these targets are broken in the Fern preview.

Problem/Use Case

Tracked as part of #1123 (Fern docs publishing support).

1. Paths like ../SECURITY.md resolve outside docs/ and are not registered in
   docs/index.yml.
2. Directory links like ./runbooks/ and ./configuration/ have no index page
   and resolve to 404 in Fern.

Copying files into docs/ was considered but rejected to avoid duplicate sources of truth. A plain text workaround was applied in #1123.

Proposed Solution

One of:

• Symlinks — symlink root files into docs/ (e.g. docs/security.md -> ../SECURITY.md).
  Git tracks symlinks; Fern follows them at build time.
• Section index pages — add docs/runbooks/index.md and docs/configuration/index.md
  as landing pages for those sections.

Component

Deployment/Config