Add two news posts (DFT+U+SOC, OMM).

[jme52]

Also add the Open Access logo.

[jme52]

The changes are already uploaded to the website:

• https://siesta-project.org/siesta/2023/02/13/DFT+U+SOC.html and
• https://siesta-project.org/siesta/2023/04/26/modular-orbital-minimization.html,

so please report any inaccuracies / suggestions for improvement sooner rather than later...

[jme52]

@albgar OK'd the new news items, so I will merge this PR.