Fix documentation browser resizing behavior

Remove hardcoded dimensions from a few CSS parameters to prevent
layout problems when readers resize their browsers to have a narrower
window.

Set a max-width: 60em on the main content so that text remains
readable on wide browser windows.

(cherry picked from commit bba7ed92a0b9800d9f5fdb14f5f66f755e9d92fa)

https://github.com/krb5/krb5/commit/0d64c9c5dccaeabc56764f026cc531f70a44bb4a
Author: Tom Yu <tlyu@mit.edu>
Commit: 0d64c9c5dccaeabc56764f026cc531f70a44bb4a
Branch: krb5-1.11
 doc/_static/kerb.css |    7 +++++++
 doc/conf.py          |    4 ++--
 2 files changed, 9 insertions(+), 2 deletions(-)