Add ability to specify README.md directory, path; fix Guide paths; fix Pages sidebar titles

1) for non node.js projects it'd be cool to specify README.md only for javascript assets (as they may be in app/assets/javascript, for example), not for whole project
2) prevent displaying of "Page" postfix in Pages section sidebar (contentPage -> content)
3) if Guide files are not in root of source folder, they are still in doc_dir/guides folder (to avoid assets missing)