Update docs-menu.css

Changed 'position: fixed' --> 'position: relative' <docs-menu #mainmenu>.

It needs more investigation because fixed positions works for core-elements docs page, but not for paper-elements docs page. Perhaps, scrollbar position is not working properly in nested situations. 

This has been noticed in Chrome and Firefox (latest versions). 

- http://www.polymer-project.org/docs/elements/paper-elements.html#paper-button
- http://www.polymer-project.org/docs/elements/core-elements.html