[Design] Left navigation changes

[KOTungseth]

We received design guidance from our Design team on the elastic.co/docs left navigation.

I've include the notes here: https://www.figma.com/board/oDOHRhyn96Pu68VxhL1iGu/Navigation-changes?node-id=0-1&p=f&t=9AI9A3YiisBuhhxB-0

Changes include:

• Make the Get started, Solutions and use cases, etc dropdown an overlay instead of the up/down push behavior (I'm sorry, Jan)
• Move section arrows to the right hand side and align as a column
• Remove the lines and spaces between the main sections
• Render the navigation text the same size, but use bold for first level titles and indentation to separate hierarchically