Break out a blog/ section of the docsite

[lihaoyi]

As discussed on the PR #3990, the page How fast does Java compile? does not really belong in documentation, and is more like a blog post. We didn't have any blog/ section before and I wanted it on mill-build.org for SEO purposes, so I just put it somewhere, but we should break out a dedicated blog/ section of the docsite for it and other articles. We can write other articles for SEO as well

Some article ideas:

• What is needed for a monorepo build tool?
• Deep dive into Mill's caching and invalidation strategy
• Deep dive into Mill's parallelism strategy
• Optimizing JVM code with JProfiler

[lefou]

Antora has a notion of modules which can be versioned. We could split up the current "ROOT" module. "blog" could be one part, the actual versioned documentation could be another. And we could keep only some basic overview stuff in ROOT, like installation, license and changelog, since those things don't really depend on the version but rather reflect the page deployment time.