perf: improve blog and homepage image performance (responsive sizes, lazy-loading, webp)

[Shivampal157]

Summary
The current site (especially the blog and homepage hero/partner logos) serves full‑size images without responsive sizing or lazy‑loading. This can slow down page loads for new visitors and mobile devices.

Motivation

• Many blog posts use large screenshots or illustrations that are always loaded at full size.
• The homepage shows several partner logos and hero imagery.
• Improving image handling will make the site feel faster and more accessible for people on slower connections, which aligns with OSL’s community and education goals.

Proposed improvements

• Enable lazy‑loading for non‑critical images (e.g. loading="lazy" on blog post images and partner logos).
• Add responsive image markup for large images (e.g. srcset / sizes or using CSS utility classes) so browsers can pick smaller images on mobile.
• Add WebP support alongside existing PNG/JPEG assets where it makes sense (keep fallbacks for older browsers).
• Optionally add a simple image utility partial in the MkDocs/Jinja theme so authors can use a consistent pattern (e.g. a macro to render blog images with lazy‑loading and alt text).

Implementation ideas

• Update the custom MkDocs theme templates in theme/ (blog post, home, partners) to:

  - Add loading="lazy" to blog content images and partner logos.
  - Use CSS classes or template logic to control image max‑width and responsive behavior.
  

• For a couple of heavy blog images and the homepage hero:
  - Generate WebP versions and reference them via or srcset while keeping PNG/JPEG as fallback.

• Document in CONTRIBUTING.md or a short “Authoring images” section how blog authors should add images (alt text, recommended dimensions, etc.).

Acceptance criteria

• Lighthouse (or similar) shows improved Largest Contentful Paint / Image elements have explicit width and height / Defer offscreen images scores for the homepage and at least one blog post.
• No visual regressions on desktop and mobile.

[Shivampal157]

Hi @xmnlab I have created this issue.
Could you please review it and assign it to me if it looks good?

[Shivampal157]

@xmnlab , plz review

[xmnlab]

Thanks @Shivampal157