SPEC4: add instruction on how to add the index server with tox.

[Carreau]

I belive

[testenv]
indexserver =
    # we use Spec 4 index server that contain nightly wheel. 
    # this will be used only when using --pre with tox/pip as it only contains nightly.
    extra = https://pypi.anaconda.org/scientific-python-nightly-wheels/simple

[Carreau]

I personally think that's one of the point where tox becomes problematic as it's becomes too hard to use/configure.

[mattip]

Could someone change the title of this to include "SPEC 04".

I believe the need for --pre comes from pip, not tox. If I understood the comment correctly.

[bsipocz]

An example for a working tox file is here: https://github.com/astropy/astroquery/blob/main/tox.ini#L26

caveats: if it's just a pip install line in the command sections then new dependencies are not being picked up. If it's an extra pip index, then however the imo silly version number requirement is need to be added, e.g. devdeps: numpy>=0.0.dev0

[bsipocz]

Shall we e.g. link the above tox file in the notes along with the other links, or you prefer to have narrative explanation in the text itself?