Improve stack readme docs

[pierskarsenbarg]

• Updated stack readme docs
• Updated stack readme example with something a bit more real world

Proposed changes

• The readme sample now has valid values in it (can't just reference resource outputs)
• Added a real world Pulumi program that matches what the sample readme has

Unreleased product version (optional)

Related issues (optional)

[pulumi-bot]

Your site preview for commit 29432b74 is ready! :tada:

http://www-testing-pulumi-docs-origin-pr-12432-29432b74.s3-website.us-west-2.amazonaws.com.

[pulumi-bot]

Your site preview for commit 5d7003da is ready! :tada:

http://www-testing-pulumi-docs-origin-pr-12432-5d7003da.s3-website.us-west-2.amazonaws.com.

[meagancojocar]

closing as I am assuming the code has gone stale, feel free to re-open once validated!