doc: use separate doctrees for distinct builders

doc: use separate doctrees for distinct builders

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 separate
doctrees was suggested. I guess there might be a slight loss of
efficiency, but it seems more robust.

[0]: build failures were first noticed in Debian experimental, but I was able to duplicate it in
     my usual build environment about 1 in 8 builds.

[1]: in particular
     https://salsa.debian.org/mpd-team/mpc/commit/9e3fc1657d043d75755993846c93f7700b97f907