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

The repository-structure block 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. The entry is moved under the
tree's existing `.github/` branch, at the block's own comment column; the
layout of the rest of the tree is untouched.

Author: hyperpolymath

.github/CONTRIBUTING.md

@@ -35,11 +35,11 @@ halideiser/
 │   └── 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