PLANET-5647: Handbook: review Pablo's recommendation #3 - Submenu block changes

[andradaradu]

Brief summary of task:

As suggested by Pablo Cuadrado and described in #49 we are exploring a more user-friendly Handbook nav.

This task is about a UX review of the Submenu block, which is widely used across the Handbook. The Submenu block is quite unpredictable having 2 columns, and the idea is to move it to the right side, made it a bit smaller, single-column, and added a "CONTENTS" title

Dependencies on other work:

• relates to #51

Help:

You can always reach out to the Planet4 team at [email protected] if you have a question. -->