feat: page-level build timings

[rich-iannone]

This PR introduces a page-level build timing feature to the documentation build system. It tracks and records how long each page takes to render during the build process, annotates which pages were served from the freeze cache, and outputs this information in a new build-timing.json artifact. A new CLI command, great-docs timing, is added to display this timing data in a human-readable table or as raw JSON, helping users identify slow pages and optimize their documentation builds.