docs: split README into landing page and per module README

Luke Hollindarequested to merge
lh-docs-readme-landing-page into master

What

Restructure the repository's documentation around the two Go modules.

  • Turn the root README.md into a landing page: what LabKit is, why there are two modules, and links to each module's README and CONTRIBUTING.md.
  • Move the previous (v1-focused) content to README-v1.md and add a notice marking v1 as security-updates-only, encouraging migration to v2.
  • Align the v2 README header with the other READMEs.
  • Point both module READMEs at CONTRIBUTING.md and drop the duplicated release-process section from the v1 README.
  • Replace the stale hard-coded reviewer list with guidance in CONTRIBUTING.md to assign a project maintainer.
  • Add a "Getting help" section (open an issue or use the #labkit Slack channel).

Why

The README was almost entirely about v1, with v2 barely mentioned. New work targets v2, so the entry point should introduce both modules clearly, steer readers to the right one, and keep contribution guidance in a single place.

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