[clang-doc] Move navbar and sidebar into normal document flow (#175110)
The navbar and sidebar caused spacing and alignment issues since they
were fixed elements with defined height/width. It was difficult to have
all elements below them correctly offset the height. This patch changes
them
to sticky elements within the normal document flow that don't define
their
height/width explicitly. This actually lets us use existing HTML/CSS
properties (like the "content" div) more naturally.
This also changes the navbar behavior to not follow while scrolling.