docs(site): simplify public documentation copy (#78)

## Summary

Make DomainKit's public documentation easier to read and keep the site,
component catalog, registry pages, and package references aligned with
the current release.

## Changes

- Rewrite homepage, quickstart, core concepts, guides, provider pages,
React docs, component pages, registry pages, and API reference copy in
plain English.
- Keep the technical names and safety rules while removing stale or
jargon-heavy phrasing.
- Align landing-page metadata and navigation copy with the current
product message.

## Tests

- `bun run --filter domainkit build`
- `bun run --filter @domainkit/react build`
- `bun run --filter @domainkit/docs build`
- `bun run --filter @domainkit/docs registry:validate`
- `bun run --filter @domainkit/docs registry:check`
- `cd apps/docs && ./node_modules/.bin/blume validate --strict`
- `bun run --filter @domainkit/docs audit --strict`
- `bun run format:check`
- `git diff --cached --check`

<!-- Screenshots omitted: copy-only documentation changes with no
meaningful layout or visual-system change. -->