Flesh out README into a landing page for plrd.org visitors

[lksbrssr]

The repo is linked from plrd.org but currently only has a 2-line README, which feels empty for visitors arriving from the site (the old protocol/research repo it replaced was a full landing/directory page).

This PR turns the README into a lightweight landing page:

• One-line mission + what PL R&D does
• The 5-stage R&D pipeline
• The four focus areas (freedom / coordination / intelligence / cognition)
• Links out to the website, blog, talks, publications, PL Neuro, and team
• The wider PL network (Protocol Labs, PL Capital, the research archive)

No code — just docs. All links verified live.

Also worth doing (needs a maintainer, can't set via PR)

• Set the repo About description + homepage to https://plrd.org
• Optionally add repo topics (e.g. protocol-labs, research, field-building)