Add announcement to docs

[mattijn]

This PR adds an announcement to the documentation page mentioning:

This website is for Vega-Altair v5 (release candidate). Visit the Vega-Altair v4 homepage for previous release documentation.

It looks as follow.

I think we will need this at least temporarily.

[joelostblom]

Good idea!

[joelostblom]

Although the tests were passing in this PR, I am seeing errors on master after merging this. They seem to be unrelated to the changes here and instead pertain to altair -saver, so maybe it was just the timing and some package like npm or similar has been updated?

[mattijn]

altair_saver is broken with npm 9, see issue altair-viz/altair_saver#113 & PR altair-viz/altair_saver#116.
It seems altair_saver requires maintenance too 😞

[joelostblom]

Hmm yeah, maybe we can merge that in. It does look like an innocent change. But I am not familiar with node at all... But at the same time we don't want to add too much maintenance if there are a lot of fixes required to altair_saver. Maybe now that vl-convert is working well it will soon be time to deprecate altair_saver? Or at least not support new releases of altair since vl-convert is a drop-in replacement?