docs: rename 'API Contract' as 'Breaking Changes' (#18399)

* Use "Breaking Changes" as the changes doc's title.

Multiple people have commented that this important page is difficult
to find, in part because people search for "Breaking" or "Changes" or
"Breaking Changes" rather than "API Contract".

This PR changes the name to "Breaking Changes" so that it'll show up
on electronjs.org/docs/.

It also adds a backwards-friendly link so people still searching for
"API Contract" will get a redirect. I'm not sure if this is necessary --
the actual URL of https://electronjs.org/docs/api/breaking-changes is
unchanged -- would especially welcome feedback on this part.

* fixup! Use "Breaking Changes" as the changes doc's title.

* fixup! Use "Breaking Changes" as the changes doc's title.

docs/api/breaking-changes.md

@@ -1,4 +1,4 @@
-# API Contract
+# Breaking Changes
 
 Breaking changes will be documented here, and deprecation warnings added to JS code where possible, at least [one major version](../tutorial/electron-versioning.md#semver) before the change is made.