docs: add category heading in docs pages

[bleucitron]

I find it useful to have the category displayed at the top of a doc page. It allows the reader to know instantly which category he/she's in.

[benmccann]

Thanks for the idea. Personally, after browsing around the deployed preview version of the site, I don't think I'm in favor of this change. I find it a bit odd to have two headings and to have a larger heading below a smaller heading.

[PuruVJ]

Im sort of in favor though. If you tweak the styles, it looks better

[bleucitron]

Your styling suggestion looks indeed much better than mine @PuruVJ