Improve style for footnotes

- More specific footnote superscript selector
- No underline for footnote/reference links
- Reference link is smaller and has padding
Use latest kramdown for table of contents
Add styles for table of contents and underline
HTPi: Add a table of contents
HTPi: Link to bluetooth setup with wifi
HTPi: Tidy up XBMC extension links
HTPi: Add xbmc advanced settings example