Website update for 5.0 release.

[jme52]

This issue is created to keep track of all the changes we need to apply to the website when we do the 5.0 release:

• Create a release news item.
• Update the text and links of the DFT+U+SOC piece of news (https://siesta-project.org/siesta/2023/02/13/DFT+U+SOC.html).