docs: Fix link and formatting in core_compatibility.rst

[eMUQI]

Description of Change

This PR introduces two small improvements to the core_compatibility.rst documentation:

• Fixes the broken link to the "Migration guide".
• Fixes the broken link to the "External Libraries Test".
• Correctly formats the ESP_ARDUINO_VERSION_STR macro as inline code for better readability.

Related links

https://docs.espressif.com/projects/arduino-esp32/en/latest/guides/core_compatibility.html

[github-actions]

	Messages
📖	🎉 Good Job! All checks are passing!

👋 Hello eMUQI, we appreciate your contribution to this project!

---

📘 Please review the project's Contributions Guide for key guidelines on code, documentation, testing, and more.

---

🖊️ Please also make sure you have read and signed the Contributor License Agreement for this project.

---

Click to see more instructions ...

This automated output is generated by the PR linter DangerJS, which checks if your Pull Request meets the project's requirements and helps you fix potential issues.

DangerJS is triggered with each push event to a Pull Request and modify the contents of this comment.

Please consider the following:
- Danger mainly focuses on the PR structure and formatting and can't understand the meaning behind your code or changes.
- Danger is not a substitute for human code reviews; it's still important to request a code review from your colleagues.
- To manually retry these Danger checks, please navigate to the Actions tab and re-run last Danger workflow.

Review and merge process you can expect ...

We do welcome contributions in the form of bug reports, feature requests and pull requests.

1. An internal issue has been created for the PR, we assign it to the relevant engineer.
2. They review the PR and either approve it or ask you for changes or clarifications.
3. Once the GitHub PR is approved we do the final review, collect approvals from core owners and make sure all the automated tests are passing.
- At this point we may do some adjustments to the proposed change, or extend it by adding tests or documentation.
4. If the change is approved and passes the tests it is merged into the default branch.

Generated by 🚫 dangerJS against 7d5e057