Document the API stability guarantee in the roadmap (#5775)

* Document what is not covered by the API stability guarantee

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Move the API stability guarantee to the roadmap

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Note that exceptions to the API stability rules are documented in the release notes

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

---------

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

.github/CONTRIBUTING.md

@@ -205,6 +205,9 @@ API of the 3.x.y version is broken. This includes:
 - Changing access specifiers.
 - Changing default arguments.
 
+What is and is not covered by this guarantee is described in the
+[roadmap](https://json.nlohmann.me/community/roadmap/#api-stability).
+
 Although these guidelines may seem restrictive, they are essential for maintaining the library’s utility.
 
 Breaking changes may be introduced when they are guarded with a feature macro such as