Clarify location of `custom.css` file

The current documentation doesn't make it clear that the path of `custom.css` file refers to a path within the `files` directory, so when I tried to create a custom CSS file it wasn't picked up.

Not sure if this change is the clearest way of expressing the correct location but it seemed perhaps better than "you can create a ``assets/css/custom.css`` file in the ``files`` directory and then..."
Further improve wording of custom.css file placement

As per: https://github.com/getnikola/nikola/pull/1512#issuecomment-64617067