feat:sideciteonce adds sice citation only once in the page

[fredguth]

closes #216

As requested, I changed the previous PR to add a new command sideciteonce and kept the current behaviour of sidecite that duplicates citation. I don't get the need for keeping sidecite previous behaviour. In my opinion repeating a citation is just wrong.

[ewu63]

Thanks for this PR! Does it work for multiple cite keys? And is it easy to extend it to other cite commands like sidetextcite?

[jboillot]

Thank you for that feature!
I did not understand why the citation keys were not flushed to the left.
The reason is that there are missing % at the end of lines, which result in breakable spots!