[chore] Add definition for documentation label

Resolves open-telemetry#27456

CONTRIBUTING.md

@@ -318,6 +318,7 @@ triaged and is ready for work. If someone who is assigned to an issue is no long
 | `bug`                | Something that is advertised or intended to work isn't working as expected.                                                                                                                             |
 | `enhancement`        | Something that isn't an advertised feature that would be useful to users or maintainers.                                                                                                                |
 | `flaky test`         | A test unexpectedly failed during CI, showing that there is a problem with the tests or test setup that is causing the tests to intermittently fail.                                                    |
+| `documentation`      | This is a collector usability issue that could likely be resolved by providing relevant documentation. Please consider adding documentation for an issue with this label.                               |
 | `good first issue`   | Implementing this issue would not require specialized or in-depth knowledge about the component and is ideal for a new or first-time contributor to take.                                               |
 | `help wanted`        | The code owners for this component do not expect to have time to work on it soon, and would welcome help from contributors.                                                                             |
 | `discussion needed`  | This issue needs more input from the maintainers or community before work can be started.                                                                                                               |