Update documentation for Byoh label usage

Description

Update test/extended/winc/README.md with documentation about the new "Byoh-" label and how to use it for filtering BYOH-specific tests.

Content to Add

Document the following:

1. *What is Byoh label*: Identifies tests that specifically test BYOH (Bring Your Own Host) functionality
2. *How to use it*:

• Use TEST_FILTERS with Byoh& to run only BYOH tests
• Example: TEST_FILTERS=Byoh&
  3. *Which tests have it*: List the 5 OCP test IDs that have Byoh prefix
  4. *When to use it*:
• Run only on Phase 1 platforms (AWS UPI, Nutanix UPI) where BYOH is supported
• Skip on IPI platforms where BYOH is not available

Example Documentation

## Test Labels

### Byoh-

The `Byoh-` label identifies tests that specifically test BYOH (Bring Your Own Host) functionality for Windows Containers.

**Tests with Byoh label:**
- OCP-42496: Configure Windows instance with DNS
- OCP-42484: Configure Windows instance with IP
- OCP-42516: Configure Windows instance with IP and DNS
- OCP-44099: Secure Windows workers username annotation
- OCP-82694: Verify containerd path removal

**Usage:**
```bash
# Run only BYOH tests
TEST_FILTERS="Byoh&" make test

# Run BYOH smoke tests
TEST_FILTERS="Byoh&;Smokerun&" make test
```

**Platform Availability:**
BYOH tests should only run on Phase 1 platforms:
- AWS UPI (platform=none)
- Nutanix UPI

Acceptance Criteria

• test/extended/winc/README.md updated with Byoh label documentation
• Documentation includes all 5 BYOH test IDs
• Usage examples provided
• Platform availability documented
• Clear explanation of what Byoh label means