www: Do not use a fixed with, but use max-width

Further:

  o ensure that the header is properly readable even on smaller screen sizes.
  o Shorten the table of contents of the documentation section.

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@197794 91177308-0d34-0410-b5e6-96231b3b80d8