Add link to main jabref homepage

[tobiasdiez]

Clicking the jabref icon in the upper left corner should direct the user to the main homepage.

[koppor]

Not yet supported by the theme. Need to submit a patch for https://github.com/cotes2020/jekyll-theme-chirpy/blob/d97f95fca0bcd450ea50709ffba0217f7e65d339/_includes/sidebar.html#L8.