Docs: close the demo-to-framework onboarding gap (link base-cli, minimal starter, docs index, skills.md) #20

Problem

Once the demo has convinced someone, there is no smooth path from "I like this"
to "I am using base-cli in my own project":

• The README never links to the framework. The only basefoundry
  repository link in README.md is ## Base →
  https://github.com/basefoundry/base (the meta-tool), at line 125. The
  base-cli repo and its docs are linked only at the very bottom of
  docs/learning-path.md (lines 106-110). Someone evaluating base-cli from this
  demo has to guess the URL.
• There is no minimal "use it in your own project" starter. Northstar is a
  deliberately production-shaped app (nested commands, fixtures, config adapter,
  optional integrations). There is no smallest possible consumer: a fresh
  project, pip install base-cli, ~10 lines wiring base_cli.App +
  one command, run it. That is what a new adopter copies first.
• skills.md is an unfilled template. It still contains the
  "## Suggested Entries" placeholder from the baseline rather than real
  repo-specific guidance.
• docs/ has no index. Six docs, linked ad hoc from README prose, with no
  "start here → next → next" reading order for an evaluator.

Suggested fix

• Add a top-of-README link block: base-cli repo, base-cli README/getting-started,
  API reference.
• Add docs/use-in-your-project.md (or a README section) with a copy-pasteable
  minimal consumer — new project, dependency line, ~10-line main, expected
  output — separate from the full Northstar tour.
• Replace skills.md placeholder content with the repo's actual workflows.
• Add docs/README.md (or a README "Documentation" list) giving an ordered
  path: why → should-I-use → five-minute learning path → lifecycle safety →
  configuration → compatibility → release.

Acceptance

• From the demo README, a reader reaches the base-cli repo in one click and has
  a minimal, runnable starter they can lift into their own project without
  reading the entire Northstar source.