Add a badge linking to DeepWiki

[niktayv]

What this PR does

DeepWiki provides a handy service for interacting with the codebase using a chat interface.
Currently, I have to request a wiki refresh manually.
Adding a badge to the README.md file will cause DeepWiki to regenerate the index automatically every week.

Test me

Open the branch add-deepwiki-badge and look at the bottom of the Getting Started section.

Checklist

• There are unit tests to verify my changes are correct or unit tests aren't applicable (if so, write quick reason why unit tests don't exist)
• I've updated relevant documentation in doc/.
• I've updated CHANGES.md with what I changed.
• I've provided instructions in the PR description on how to test this PR.