Allow building Sphinx docs

[jsquyres]

If docs/requirements.txt exists in the source tree, that means that we
have Sphinx documentation, not Pandoc documentation. Create a Python
virtual environment before running configure and install the Right
things for Sphinx to build the man pages. This also means that there
will be no Pandoc-generated man pages, so don't bother trying to
convert them to HTML.

Signed-off-by: Jeff Squyres [email protected]

[jsquyres]

@bwbarrett I'm not entirely sure that this is correct; I could use your guidance here. The intent for this PR is to support open-mpi/ompi#8329 by allowing building the Sphinx docs if they are present (and skip doing some Pandoc things). Once open-mpi/ompi#8329 merges, we can further simplify a few things here in ompi-scripts by fully removing all Pandoc things.

[bwbarrett]

We run CI tests on branches prior to 5.0.x, so we can't remove this block for some time. That said, it needs to change. I think the logic becomes:

1. if no --disable-man-pages option to configure, do nothing. (this is a pre-pandoc build)
2. if docs/requirements.txt exists, do nothing (this is a 5.0.x or later build, so configure will autoselct forward whether or not Sphinx is found).
3. if pandoc is found, do nothing (this is a pandoc-era build, and we have pandoc)
4. else, add --disable-man-pages to configure (this is a pandoc-era build, and we don't have pandoc, so we need to add a configure argument to let the build succeed).