Refresh the module reference landing page

[CoreyRDean]

Non-technical summary

• Replaced the legacy docs/reference.md dump with a current reference landing page for the live RCCE source tree.
• The docs now show where the documented modules fit into Client, Server, and GUE, and they stop implying that undocumented Project Manager/framework surfaces are already covered.
• This matters now because docs/reference.md is a top-level entry from both the README and docs index, so stale structure there makes contributor orientation harder right after the onboarding refresh.

Technical summary

• Rewrote docs/reference.md around the current application entrypoints in src/Client.bb, src/Server.bb, src/GUE.bb, and src/Project Manager.bb.
• Added an application map for Client, Server, GUE, and Project Manager that reflects the documented module surface used by the live tree.
• Added a normalized-name note for ClientAreas_FE.bb -> docs/modules/clientareas.md so the page matches current source naming.
• Replaced the old flat list with grouped module indexes for gameplay/content, interface/assets, and networking/services.
• Added an explicit Current Gaps section covering still-undocumented framework, graphics, and helper modules instead of leaving that missing coverage implicit.
• Validation: git diff --check plus a local markdown-link resolution check for every repository-local link in docs/reference.md.
• Breaking changes: none.

Additional notes

• Trade-off: this keeps the increment bounded to the docs landing page instead of widening into rewriting the individual module pages.
• Follow-up intentionally deferred: dedicated reference pages for Project Manager/framework modules, SpawnTracking.bb, and other currently source-only helpers.
• Remaining gap: the repo still has undocumented runtime/framework modules; this PR makes that gap visible and navigable, but it does not close it.