[PATCH] doc: use separate doctrees for distinct builders

[David Bremner]

David Bremner <david at tethera.net> writes:

> It seems our previous attempt with order-only targets was not
> sufficient to avoid problems with sphinx-builds doctree cache [0].
> Looking around at other people's approaches [1], using seperate
> doctrees was suggested. I guess there might be a slight loss of
> efficiency, but it seems more robust.

pushed to release (soon to master), with Tomi's spellink fichs