docs: website: more header spacing tweaks

docs: website: more header spacing tweaks

- remove top padding for h1. this means page titles sit flush with the top
  of the side bar (like 'The virtualization API' on the front page)
- up the top padding for the remaining sections. makes it visually easier
  to tell adjacent header sections apart, especially in dense wiki pages
- use two different spacing levels for h2-h4 and h5-h6,
  gives pages some more visual flexibility
- use a slightly lower bottom padding... this makes top padding stick out
  more which makes it visually easier to differentiate between adjacent
  header sections