Add walnut to developer tools

[marijamijailovic]

This PR adds the Walnut transaction debugger and simulator to the Developer Tools section.

Checklist

• The URL is not already present in the list (check with CTRL/CMD+F in the
  raw markdown file).
• Each description starts with an uppercase character and ends with a
  period.
  Example: cairo - The Cairo programming language.
• Drop all A / An prefixes at the start of the description.
• Avoid using the word Starknet in the description.

[mazurroman]

gm @AbdelStark can we ask for review here