Auto-generate dotnetup documentation walkthrough from executable output

## Summary

The dotnetup preview documentation (added in #54412) includes a hard-coded walkthrough of the interactive setup flow. This walkthrough will need to be updated every time the CLI workflow changes, creating a maintenance burden.

## Proposal

Add a mechanism to automatically generate or ingest the dotnetup walkthrough content into the documentation. For example:
- Have the build run the executable at the end and dump the interactive output into the README/docs
- Or provide a script/target that regenerates the walkthrough section from a recorded session

This would reduce the maintenance burden of keeping docs in sync with CLI changes.

## Context

From the PR #54412 review discussion:

> I really like this inclusion of the walkthrough, though I am worried about the maintenance burden of updating this every time we change the workflow. Should we mention this in copilot instructions somewhere at minimum?
> I wish there was an 'easy' way to ingest this into the readme... such as having the build run the executable at the end and dump the content into the readme but that can definitely be a 'future' thing as a follow up issue

For the internal and public previews, hard-coding is acceptable, but longer-term an auto-update mechanism would be ideal.

/cc @baronfel @nagilson