Docs Design: Decreasing the line-height on the left-side menu in the Api Docs

[peterpme]

After experimenting around a bit I felt like having the line-height smaller (putting every item closer together) was easier to read on the eyes.

The suggested differences:

before (current way):

after:

[chenglou]

Finally, design-minded issues... thanks!

[ryyppy]

the layout will be revisited (yet again) when we allocated new design resources somewhere in March 2021

[fhammerschmidt]

this looks better now imo