[Docs] Feature overview with GEMMA/TEC standards compliance

[rubenvdlinde]

Create docs/features/README.md with:

• Standards compliance table (GEMMA referentiecomponenten, TEC RFP sections, Forum Standaardisatie)
• Feature index table (all implemented features with summary, standards refs, doc links)
• Individual feature docs for each major feature group

Part of org-wide effort to document all Conduction apps with standards references.