docs: restructure docs/ to be service-centric with README linking out to guides

[ecv]

Summary

The README has become the catch-all for anything a reader might need — product overview, prerequisites, local development loop, deploy/uninstall instructions — while docs/ holds the actual reference material (API pages, runbooks, enhancements, testing guides) that nothing links to. A contributor landing on the repo reads a wall of make invocations before learning what the service does, and an operator looking for a runbook has no path to it from the front page.

Restructure so the README is service/product-centric and links out to task-specific guides under docs/.

What the README should be

• What Datum Network Services is, and the boundary with infrastructure providers (already there — keep it)
• The resource model at a glance: Networks, Network Contexts, Subnets, HTTPProxies, Domains, TrafficProtectionPolicies
• A short "where to go next" index into docs/ — one line per destination, no procedures inline

What should move to docs/

• Running locally for development (added in docs: add running the operator locally guide #310) → docs/development/running-locally.md
• Prerequisites → same developer guide, not the front page
• Deploy on the cluster / Uninstall → docs/deployment/ (or fold into an existing operator-facing page)

Also worth doing while in there

• docs/ has no index. Add docs/README.md grouping the existing trees (api/, runbooks/, testing/, enhancements/, diagrams/) so the README can link to one place instead of eleven.
• The README's Documentation section still says docs.datum.net is coming "shortly." Either point at what exists today or drop it until the site is live.

Scope note

Docs-only reorganization — no content rewrites beyond what relocation requires. Keep each fact owned once: if a page moves, leave a link, not a copy.

Raised by @scotwells in review of #310.