Rewrite README with a product focus

[dinooo13]

Summary

Rewrites the README so it leads with what the site is and who it serves, rather than the stack.

• What this is / who it's for: the site's purpose (who I am, what I build, where I speak) and its audiences (organisers, engineering leaders, engineers, recruiters)
• What's on the site: Home, Labs (with current labs), and Speaking, described as features
• Product principles: fast, accessible, findable, dark-first/light-ready, content over code
• Updating content: a table mapping each part of the site to its YAML file, with a pointer to CLAUDE.md for the full field reference
• Running it locally / how it ships / built with: the developer setup, condensed and moved to the end

Docs only. No site code changes.

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