docs: Add icons for boxd, exe.dev, islo, and mirage

[corymsmith]

• Add icons for sandboxes without icons (boxd, exe.dev, islo, and mirage)
• Extracts svgs out to a new file to clean up the ecosystem page
• Removes smolvm for now since the link 404's as its not currently available (see Add smolvm sandbox connector #101)
• Uses the sandbox / ecosystem items with icons on the homepage.

[astrobot-houston]

Thanks for the contribution! We're closing this PR and moving the conversation to the discussion: #299

We've moved to a model where bugs and feature proposals are discussed in issues/discussions before code review, so the community can help prioritize and shape the work. Your branch is linked from the new thread so the implementation isn't lost — please join us there to continue the conversation.

— astrobot 🤖