docs: prepare v0.4.0 release

Rewrite the README around the supported public API, add a public API
stability policy, document the deprecation timeline, and refresh the
CHANGELOG with the full 0.4.0 scope. Add a short CONTRIBUTING guide.
Apply minor pending fixes to LICENSE and .env.example. Fix a couple of
verify=False antipatterns in the docs/ pages.

README:
- Badges (PyPI version, Python versions, license, CI).
- Quick start that actually runs (no missing import, no global
  disable_warnings, correct VYDEVICE_VERIFY_SSL parse).
- Environment variable table with defaults.
- ApiResponse contract documented as a dataclass.
- Compact API overview (configure / retrieve / show / generate / reset /
  config-file / system / image).
- Public API stability section with the 0.4 -> 0.5 -> 0.6 -> 1.0
  deprecation timeline.
- Logging section: NullHandler default, pyvyos logger, redacted key.
- VyOS compatibility note (1.4 LTS, 1.5 rolling).
- Development with uv; optional pre-commit.

CHANGELOG:
- Consolidated 0.4.0 entry covering both the architecture refactor and
  this cleanup, since 0.4.0 was tagged in git but never published.
- Explicit Added / Changed / Removed / Fixed / Compatibility / Notes
  sections.
- 0.3.0 entry annotated as 'tagged but never released to PyPI'.

CONTRIBUTING.md:
- Scope statement (thin HTTPS API wrapper).
- Pull request rules; explicit 'do not change HTTP payload in passing'.
- Development setup with uv.
- Public API stability summary.

Fixes from the previous review-fixes branch:
- LICENSE copyright now reads 'GravScale, Roberto Berto'.
- .env.example default flipped to VYDEVICE_VERIFY_SSL=true with
  field descriptions.
- docs/getting-started.md: VERIFY_SSL default to true; urllib3
  disable_warnings now passes the specific InsecureRequestWarning.
- docs/index.md: quick example uses verify=True; removed RTD link
  (RTD config was removed in this release because the build never
  worked).

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