[Mega Menu UI] `MegaMenuDropdown` icon spacing and position

[techtolentino]

Steps to reproduce the problem

Have a look at the MegaMenuDropdown on the Feature Learn Content branch

What is the current behavior?

• the down chevron icon isn't vertically centered
• there's not enough spacing/padding, to the right of the down chevron icon

What is the expected behavior?

• centered vertically (like qiskit.org/textbook-beta shows today)
• slightly more spacing to right of the icon

Screenshots

CC: @JRussellHuffman