Doujinshi IS a manga, except it's a fanart manga... Which means it's a manga that is not drawn by the original artist, or that is based on a book/anime/story but with changed plotline... ^^;; Hope that explains it...
And I don't think they scanlate Harry Potter doujinshi because most doujinshi is in Japanese and is only for sale, NOT for free scanlation... And they sell it online, or at some other Japanese stores with doujinshi labels.
Most HP doujinshi are hentai or yaoi ^^;