.Net: Create a getting started project for vector stores.

[westey-m]

Motivation and Context

#7606

Only three steps so far, more to be added in a subsequent PR.

Description

• Add a getting started project for vector stores
• Add three initial steps, more to follow
• Update README files

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄