docs: Make most headers a bit smaller
docs: Make most headers a bit smaller

Headers are bold already, so the font doesn't need to be that
big to draw attention.

datto/libvirt,taget/libvirt,zippy2/libvirt,libvirt/libvirt,crobinso/libvirt,jfehlig/libvirt,fabianfreyer/libvirt,nertpinx/libvirt,crobinso/libvirt,datto/libvirt,eskultety/libvirt,eskultety/libvirt,crobinso/libvirt,jardasgit/libvirt,olafhering/libvirt,datto/libvirt,jardasgit/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,fabianfreyer/libvirt,VenkatDatta/libvirt,andreabolognani/libvirt,andreabolognani/libvirt,zippy2/libvirt,VenkatDatta/libvirt,olafhering/libvirt,libvirt/libvirt,andreabolognani/libvirt,nertpinx/libvirt,libvirt/libvirt,taget/libvirt,nertpinx/libvirt,crobinso/libvirt,VenkatDatta/libvirt,olafhering/libvirt,zippy2/libvirt,VenkatDatta/libvirt,andreabolognani/libvirt,zippy2/libvirt,eskultety/libvirt,olafhering/libvirt,jardasgit/libvirt,nertpinx/libvirt,fabianfreyer/libvirt,nertpinx/libvirt,datto/libvirt,datto/libvirt,taget/libvirt,libvirt/libvirt,eskultety/libvirt,VenkatDatta/libvirt,andreabolognani/libvirt,fabianfreyer/libvirt,eskultety/libvirt,taget/libvirt,jfehlig/libvirt,jfehlig/libvirt,taget/libvirt,jfehlig/libvirt,jardasgit/libvirt