Add 5.0 NEWS to RST docs

[jsquyres]

The contents of #10079 were pretty much overwritten immediately with the RST docs.

The content from that PR needs to be added to the new docs/ tree -- a new news/news-5.0.x.rst file is likely needed, similar to the news files for the other release series.

[gpaulsen]

I'll do it tomorrow. thanks

[gpaulsen]

@awlauria posted #10084