Expand the "Handling compatibility breakages" section

[southpawriter02]

The "Handling compatibility breakages" section in engine/guidelines/index.rst had a TODO asking to elaborate on the types of compatibility and procedure. The only other content was a bare link to the external docs page. This meant a contributor landing here got no orientation before being sent elsewhere.

This fills in that section with a brief overview:

• What each release level (patch, minor, major) promises about compatibility
• The distinction between binary and source compatibility
• When and why GDExtension compatibility methods are required
• What the CI validation catches and what the error looks like
• A note about the "breaks compat" label and review expectations

The external docs link is preserved at the bottom for anyone who needs the full technical walkthrough. The section is meant to orient contributors, not replace the external page, so it stays high-level on purpose.

[Ivorforce]

LGTM

Before we can merge, please squash your changes down into a single commit.

[Calinou]

Looks good to me.