Remove fixed font-size

[winstonyin]

• Visual hierarchy of the h1, h2, etc. elements is important. I remove the 17px fixed font-size for section title elements and adjust code font-size to a relative 92%. I also adjust the spacing a little.
• Mention mathlib4 on the home page. Add link to point to community page and mathlib3 docs.
• The biggest text on the top left should be Mathlib4 rather than the non-descriptive Documentation. Size adjusted to h2.
• The file name shown at the top is now grey to reduce visual fatigue. Size adjusted to h2, centre aligned. Line height changed to 95% to fit better when wrapped.
• Fixed a couple typos, where .header is used in the CSS selector instead of header.
• Replace the repeated file name on the top right with a "return to top" link.

[eric-wieser]

• The biggest text on the top left should be Mathlib4 rather than the non-descriptive Documentation. Size adjusted to h2.
• The file name shown at the top is now grey to reduce visual fatigue. Size adjusted to h4

h1-h6 shouldn't be chosen based on size, but on hierarchy. Size is a style consideration best left of CSS

[hargoniX]

Apart from the mathlib specific changes this looks fine to me (but feel free to bikeshed on frontend as much as you like, I have no idea what's correct here). As for my opinion on the mathlib specific changes, I already explained that over here yesterday: https://github.com/leanprover/doc-gen4/pull/172#issuecomment-1879116753