Make it easier to re-use a search service

[pamelafox]

Purpose

This PR makes it easier to re-use a search service, by making it possible to specify everything via azd env set and writing it all out clearly in the README.

Hopefully helps with #586

Does this introduce a breaking change?

[ ] Yes
[X] No

Pull Request Type

What kind of change does this Pull Request introduce?

[ ] Bugfix
[X] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Documentation content changes
[ ] Other... Please describe:

How to Test

• Try a new azd up with existing search resource, following the README instructions
• Also try a standard azd up with all new resources