docs: add known issues documentation for readOnly fields

[1234-ad]

Description

This PR addresses issue #2722 by adding documentation for the known issue with readOnly fields appearing in code examples where they shouldn't.

Changes

• Added KNOWN_ISSUES.md file documenting the readOnly fields problem
• Provides clear explanation of the issue with examples
• Offers practical workarounds for users affected by this issue
• References the specific issue readOnly fields are displayed in the code examples for PATCH requests #2722

Why this change?

Issue #2722 reports that readOnly fields incorrectly appear in PATCH request examples. While the core issue requires code changes to the example generation library, this documentation:

1. Acknowledges the problem - Validates user reports and shows awareness
2. Provides immediate help - Offers actionable workarounds users can implement now
3. Reduces support burden - Prevents duplicate issue reports
4. Improves user experience - Users can continue working while waiting for a fix

Benefits

• Users can immediately resolve their documentation issues using the workarounds
• Clear examples show exactly how to implement the solutions
• Reduces frustration for users encountering this common issue
• Provides a reference point for similar schema-related problems

Workarounds Provided

1. Manual Examples - Explicitly define examples excluding readOnly fields
2. Schema Duplication - Create separate read/write schemas for different operations

This is a documentation-only change that provides immediate value while the underlying issue is being investigated.

[RomanHotsiy]

I created a proper fix here: Redocly/openapi-sampler#183