Add pages for often-searched questions

[tobiasdiez]

Based on the Google Analytics report and what kind of questions get often clicked on in the forum, the following points should be addressed (in more detail) in the documentation:

• "doi to bibtex", "doi to bib", "isbn to bibtex", "bibtex download" (these are already covered, just add these phrases to the help page to get a higher ranking in google)
• bibtex vs biblatex
• bibtex to word, jabref word
• bibliography latex