AutoRest emitter should generate readme.md as output

## Background

Today, readme.md is still actively used by downstream tooling and pipelines(RpaaS, docs) and service teams have to hand-author and hand-update it whenever their TypeSpec changes.
Since we treat TypeSpec as the single source of truth, this manual step is redundant and creates gap between TypeSpec and the generated AutoRest configuration.

## Proposal

The AutoRest emitter (typespec-autorest) should emit readme.md as part of its standard output. It shall contain all the TypeSpec definitions and emitter configuration so downstream partners keep working without changes
It will eliminate the manual maintenance burden for service teams, keep readme.md in sync with TypeSpec automatically, and reinforce TypeSpec as the single source of truth.

Start at the typespec-autorest emitter entry point and trace how its standard output is produced. Compare that output with the readme.md expected by RpaaS, docs, and service pipelines; done means the emitter generates a readme.md containing the TypeSpec definitions and emitter configuration without manual updates.

Written by the indexing model from the issue text.