docs: refresh readme & add sponsors update workflow

[CodeDredd]

README

• Rewritten for 2.0.0: short feature overview, a real quick-start (install + model definition in the new standard-decorator syntax + query), requirements matrix (pinia/Node/TS per major)
• The vuex-orm comparison table moved into the docs (what-is-pinia-orm) and got updated on the way (morphToMany/morphedByMany are ✅ since long, decorators row now says standard decorators, migration-guide link)
• Fixed the broken LICENCE link (file is LICENSE)

Sponsors

New sponsors.yml workflow: runs monthly (and via workflow_dispatch) and regenerates docs/public/sponsorkit with sponsorkit, committing with [skip-release].

⚠️ Setup needed once: create a repo secret SPONSORKIT_GITHUB_TOKEN — a classic PAT with read:user + read:org scopes (the Actions GITHUB_TOKEN can't read sponsorships).

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.24%. Comparing base (b414611) to head (37b706c).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2049   +/-   ##
=======================================
  Coverage   99.24%   99.24%           
=======================================
  Files          83       83           
  Lines        3185     3185           
  Branches      562      562           
=======================================
  Hits         3161     3161           
  Misses         17       17           
  Partials        7        7           

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