Convert images to webp and add scripts to keep doing so

[LukasWallrich]

On reflection, it makes little sense to convert to webp on GitHub - there might be exceptions to handle. Instead, this PR converts all jpg/png files to webp (faster and smaller images), adds a script to do such conversions locally when needed, and adds an action that highlights the potential need to convert to webp on PRs that contain pictures - but leaving it up to the contributor / reviewer whether to bother with that at this moment.

[github-actions]

⚠️ Image Files/References Detected

Image Files not in ignore list:

• scripts/webp_conversion/picture_archive/content/about/partnerships/cos.png

Image References not in ignore list:

• content/about/partnerships/partnerships.md: cos.png

Please consider converting these images to WebP format and updating references accordingly.

[github-actions]

👍 All image files/references (if any) are in webp format, in line with our policy.

[LukasWallrich]

@flavioazevedo after you probably received a ridiculous amount of notifications from this, it is now good for review. Converting the images to webp will make the webpage a bit faster and smaller, and meet a grant objective.

[flavioazevedo]

I cannot stress how amazing this is @LukasWallrich ! REALLY, this is freaking awesome! WOW! THANK YOU!