Website update for 6.0 release

[jme52]

This issue is created to keep track of all the changes we need to apply to the website when we do the 6.0 release:

• Create a release news item.
• Update the text and links of the last paragraph of the libOMM news item (https://siesta-project.org/siesta/2023/04/26/modular-orbital-minimization.html).