Add PySM methods ApJ publication post

[zonca]

Summary

• add a new group post announcing the PySM methods paper publication in ApJ
• include the shared image asset and reference open resources

Testing

• bundle exec jekyll build

[seclark]

This looks great @zonca, thank you! My only suggested change: call the arxiv version "the arxiv version" instead of the "open access" version. The paper is published as an Open Access ApJ paper :)