Add documentation and examples for multiple section types (p2, r2, m2, e2)

[Copilot]

Casbin-rs already supports multiple section types via EnforceContext and add_named_policy, but this wasn't documented, making the feature invisible to users looking for parity with Go/Node.js/Java implementations.

Changes

• README.md: Added "Multiple Section Types" section with model configuration and usage examples
• README.md: Added entry to examples table linking to existing multi_section_model.conf and multi_section_policy.csv
• src/enforcer.rs: Added test_multi_section_model covering section enforcement and policy management

Usage

use casbin::prelude::*;
use casbin::EnforceContext;

let e = Enforcer::new("examples/multi_section_model.conf", "examples/multi_section_policy.csv").await?;

// Default section (p, r, m, e)
e.enforce(("alice", "read", "project1"))?;

// Second section (p2, r2, m2, e2)
e.enforce_with_context(EnforceContext::new("2"), ("james", "execute"))?;

Model definition supports multiple sections:

[request_definition]
r = sub, act, obj
r2 = sub, act

[policy_definition]
p = sub, act, obj
p2 = sub, act

[matchers]
m = r.sub == p.sub && r.act == p.act && r.obj == p.obj
m2 = r2.sub == p2.sub && r2.act == p2.act

Original prompt

---

This section details on the original issue you should resolve

<issue_title>rust multiple section types example</issue_title>
<issue_description>I found multiple section types example with go、node 、java. but can't find rust, does rust support multiple section type?
https://casbin.org/docs/syntax-for-models#multiple-section-types</issue_description>

Comments on the Issue (you are @copilot in this section)

@hsluoyz need to support `p2` like Golang Casbin

Like Node-Casbin's issue: apache/casbin-node-casbin#472

refer to PR: apache/casbin-node-casbin#518</comment_new>

• Fixes rust multiple section types example #318

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[github-actions]

Benchmark for 3c53722

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Test	PR Benchmark	Master Benchmark	%
b_benchmark_rbac_model_large	9.7±0.05ms	9.6±0.04ms	+1.04%
benchmark priority model	1390.9±20.89ns	1364.0±23.64ns	+1.97%
benchmark_abac_model	1108.6±16.09ns	1104.6±10.13ns	+0.36%
benchmark_basic_model	1200.4±39.95ns	1159.7±11.25ns	+3.51%
benchmark_key_match	4.0±0.08µs	4.0±0.07µs	0.00%
benchmark_raw	0.0±0.00ns	0.0±0.00ns	NaN%
benchmark_rbac_model	3.0±0.05µs	3.2±0.09µs	-6.25%
benchmark_rbac_model_medium	794.9±5.06µs	793.0±5.13µs	+0.24%
benchmark_rbac_model_with_domains	1963.4±31.34ns	1967.9±39.91ns	-0.23%
benchmark_rbac_with_deny	5.3±0.09µs	5.2±0.08µs	+1.92%
benchmark_rbac_with_resource_roles	1373.7±21.35ns	1422.2±20.88ns	-3.41%
benchmark_role_manager_large	4.3±0.03ms	4.3±0.01ms	0.00%
benchmark_role_manager_medium	268.0±4.94µs	268.5±2.11µs	-0.19%
benchmark_role_manager_small	85.4±1.31µs	83.8±1.23µs	+1.91%