Docs: fold Fog family into root and index READMEs

[mdheller]

Summary

This PR completes the fog documentation cleanup in sourceos-spec by folding the already-merged FogVault / FogCompute family into the canonical index/readme surfaces.

It updates:

• README.md
• schemas/README.md
• examples/README.md
• semantic/README.md

Why

The fog schema family, patch fragments, and semantic seed are already on main, but they were still easiest to discover through dedicated additive files rather than the repo’s canonical index surfaces.

This PR makes the fog layer visible where readers actually look first:

• root repo overview
• schema catalog
• examples index
• semantic overlay index

Included in this PR

• add Fog Layer row to the root schema family table
• mention fog patch fragments and semantic vocabulary seed in the root layout/overview
• add fog section and fog entries to the schema catalog
• add fog examples to the examples index
• add fog vocabulary seed to the semantic overlay index

Non-goals

• no schema changes
• no OpenAPI / AsyncAPI changes
• no semantic model changes beyond documentation/index visibility