docs: 1.9.0 docs - README, info.md, trimmed changelog, GPL-3.0

- README rewritten around the 1.9.0 line: what 1.9.0 changes, the ramp guard in
  the control law, the two dashboards, the new helpers, and corrected counts
  (91 templates, 25 helpers, 9 entities).
- info.md added: the HACS store page for the current release.
- CHANGELOG.md now documents the current release line only; the notes of 1.8.0
  and older moved to the wiki (Changelog archive).
- Licence corrected to GPL-3.0 (README badge and licence section, info.md).
  LICENSE has always carried the GNU General Public License v3; the documents
  claimed MIT.

Author: acdcnow