docs: #2380 fix stale README documentation links and keeperhub/ paths

[subheeksh5599]

Fixes #2380.

Repoints the three Documentation links that 404 on staging, and drops the keeperhub/ prefix from the three places that carry it. There is no top-level keeperhub/ directory in the tree.

Documentation links (all three returned 404):

• docs/getting-started/quickstart.md -> docs/getting-started/index.md
• docs/intro/concepts.md -> docs/concepts.md (there is no docs/intro/ folder; the page is at the docs root)
• docs/workflows/examples.md -> docs/workflows/index.md (the workflows folder has creating, templating, schema-reference, import-export, hub and marketplace)

The replacement targets match the docs nav: docs/_meta.ts maps concepts to "Core Concepts" and docs/getting-started/_meta.ts lists index as the Overview.

keeperhub/ prefix (nonexistent path), three places:

• README.md:197 Services table, App source: app/, keeperhub/ -> app/. The App is the Next.js application and app/ is its directory; the second path never existed.
• README.md:228 Plugin System sentence: `keeperhub/plugins/` -> `plugins/`
• README.md:262 Metrics Reference link: keeperhub/lib/metrics/METRICS_REFERENCE.md -> lib/metrics/METRICS_REFERENCE.md (the file exists at that path)

Verification on the branch:

• All seven relative links in the README resolve against the tree (checked each with git cat-file -e; the four that were broken now resolve, the three that were already fine are unchanged).
• No keeperhub/ reference remains in the README outside the real keeperhub-* service directories.
• npx tsx scripts/check-api-docs-routes.ts reports no docs-vs-code drift.
• Type-check and lint are unaffected (README only; Biome does not lint markdown).

Docs only, no code change.