Plugin manual - Added CSS, improved HTML format

- Added a CSS style sheet to plugin manual folder
- Revised HTML formatting, by removing (almost) all hard-coded formats / styles and implementing them on the CSS side
- Added some styles to improve property table readability
- Added a logo (via CSS) and a footer to each page
- No change to the code logic.
Merge pull request #664 from mgavioli/Plugin_manual_css_and_html_styles

Plugin manual - Added CSS, improved HTML format