Add supporters section and refresh contributor guide

[gilgamezh]

Summary

• replace the community contributors block with a supporters & sponsors section for Gandi.net, the PSF, and Zammad
• add higher-resolution branding for Gandi and include the PSF logo in the layout
• update AGENTS.md so contributors use the shared .venv before running commands

Testing

• source .venv/bin/activate && make html