[ticket/12599] Move documentation stylesheet to assets directory.

PHPBB3-12599
[ticket/12599] Move images to assets/ and update the phpBB logo.

PHPBB3-12599
[ticket/12599] Use the back to top icon instead of text.

PHPBB3-12599
[ticket/12599] Add padding on the sides of the inline code boxes.

PHPBB3-12599
[ticket/12599] Get the font sizes under control.

PHPBB3-12599
[ticket/12599] Place standalone text in a box.

PHPBB3-12599
[ticket/12599] Remove empty line at end of code boxes.

PHPBB3-12599