Add documentation for using lifecycle (EOL) annotations

[lbussell]

Describe the Problem

Image lifecycle annotations are being added to images as part of Add lifecycle annotations to images pushed to MCR (docker-tools#1201).

Describe the Solution

We should have guidance in this repo's documentation for using these annotations. It should probably include:

1. How to determine if a given image is out-of-date/EOL using lifecycle annotations
2. How to determine when the image went out of date/when the annotation was created
3. How to find the latest image if you are referencing an EOL image by its digest

Additional Context

Helpful link for whoever documents this: CSSC [MSFT internal link]