Make root PHILOSOPHY.md a symlink to the docs philosophy page
The root PHILOSOPHY.md had drifted into a stale duplicate of
docs/source/en/conceptual/philosophy.md (which was rewritten in #13808 to
cover the single-file model policy and Modular Diffusers). Replace it with a
symlink to the docs file, mirroring CONTRIBUTING.md -> contribution.md, so
there's a single source of truth.
A symlink renders as a path stub (not content) on GitHub, so repoint the
links that pointed at blob/main/PHILOSOPHY.md to the rendered docs site:
the PR template and the English contribution guide.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
