Docsite: Opal docs, client bundle perf, devlog sidebar

Added Opal documentation page with atUri frontmatter. Eliminated a 1.3MB client bundle by removing markdown and AT Protocol dependencies from the client-side code — these were only needed server-side for content rendering. Added devlog link to the sidebar navigation.

Also added and then reverted a Sequoia documentation page — docsite is for own projects only, not third-party tools.