Add release instructions

[jhale]

Probably won't get all of this done in one go but it's a start.

[jhale]

The release branches are in a reasonable state for finalising a release.

[chrisrichardson]

Another thing to mention, is that whilst the integration testing worked smoothly, the tags did not build the documentation the first time, because the CI was pointing to main and the compatibility requirement failed. I changed the CI to look at release branch for the dependencies, then it worked. It may be better (perhaps) to update the versions in main before tagging?

[jhale]

Thanks for the explanation! I see the issue, I'll add something to the documentation.

In the content of DOLFINx, it would be great if the Actions simply read a JSON file with the git refs of the accompanying FEniCSx components, rather than having to change them manually in about 10 files.

[nate-sime]

Should some details about adding a new version to the Spack packages be added in a future PR?