docs: add authorization matrix

[h1065153539-create]

Summary

• add docs/AUTH_MATRIX.md mapping contract entrypoints to required roles and signer checks
• group admin, business, investor, dispute, public read-only, and maintenance-style entrypoints for contributor review
• link the new matrix from the top-level README

Verification

• Documentation-only change; no code paths modified.
• Checked the matrix against quicklendx-contracts/src/lib.rs and auth helpers such as AdminStorage,
  equire_auth, PauseControl, and verification helpers.

Closes #1479

[h1065153539-create]

CI note: the �uild check is failing, but the failure is not caused by this docs-only change.

Evidence from the job log:

• The workflow received 2 changed files: README.md and docs/AUTH_MATRIX.md.
• It printed: Docs-only PR detected; skipping Soroban contract build steps.
• The later cargo test --verbose step fails while compiling the existing crate with �rror: this file contains an unclosed delimiter at src/lib.rs:94:30.
• The current base main SHA dc8f49d already has a failing �uild check with the same source-level failure pattern.

I kept this PR limited to documentation and README discoverability for #1479.