Bringing controls from discover to dashboard now also works by saving the visualization

[florent-leborgne]

This PR:

• Documents that adding Discover's visualization to a dashboard using the Save visualization option now also brings any existing variable controls (used in the corresponding ES|QL query) along. This is currently live in serverless and will also come with 9.3. Previously, only saving the discover session and then adding it from the library could work.
  To make this clearer, I split the current section into 2 "methods".

Closes: #3992

Summary

Generative AI disclosure

1. Did you use a generative AI (GenAI) tool to assist in creating this contribution?

• Yes
• No

[github-actions]

✅ Vale Linting Results

No issues found on modified lines!

[stratoula]

Yaaaay! Looks great, thanx for adding it Florent!

[nastasha-solomon]

Looks good! Small, non-blocking nits about the screenshot: one annotation is prob fine. The color should also be from Elastic's brand colors. Style guidance on screenshots:

If you need to draw attention to a specific part of the screenshot, you can draw a border or use an annotation, such as an arrow. If you do, keep these in mind:
Use Elastic's brand colors for the annotation. We recommend the pink accent color, hex #F04E98.