fix(docs): list the contributing guide where it actually lives (#56)

The "Repository Structure" block still listed `CONTRIBUTING.md` at the
repository root, but the guide lives at `.github/CONTRIBUTING.md` — the
entry pointed at a location it had already moved away from.

This is the only change: the entry moves under the tree's existing
`.github/` branch, at the block's own comment column. The tree already
rendered correctly and its layout is untouched.

Author: hyperpolymath

.github/CONTRIBUTING.md

@@ -39,11 +39,11 @@ anytype/
 │   └── contractiles/    # Policy contracts (k9, dust, lust, must, trust)
 ├── .well-known/         # Protocol files (Perimeter 1-3)
 ├── .github/             # GitHub config (Perimeter 1)
+│   ├── CONTRIBUTING.md  # This file
 │   ├── ISSUE_TEMPLATE/
 │   └── workflows/
 ├── CHANGELOG.md
 ├── CODE_OF_CONDUCT.md
-├── CONTRIBUTING.md      # This file
 ├── GOVERNANCE.md
 ├── LICENSE
 ├── MAINTAINERS.md