Updates to packaging and installation

[jhale]

This PR removes the use of the latest tag. The latest tag has no clear meaning. Instead, images will either be tagged nightly, stable or specifically versioned e.g. v0.4.1. All public facing documents will prioritise the use
of the explicit stable tag. There will be further updates to be made in some of the other FEniCS components CI.

This also harmonises installation instructions in README.md.

[jhale]

I will probably have to do this commit as two stages to stop the CI breaking overnight.

[francesco-ballarin]

@jhale can you please login to docker hub and manually remove the latest docker tag, so that people realize that they must use the nightly tag? Related to this: what will happen if one uses docker pull dolfinx/dolfinx without any tag, and without latest being available? An error? latest will redirect to nightly?

(EDIT: sorry, I hadn't seen the message on slack concerning the removal of latest in 30 days)

[jhale]

No problem, I should have posted it here too!

The :latest tag will be removed in 30 days.

It is true we will no longer have a default tag :latest. I'd rather not point :latest at :nightly because then we will start having end users using :nightly with all of the issues that brings with support. I would rather stick with the new explicit design. If not having a :latest tag causes lots of issues we can revisit the issue at a later date.