[chore] Add definition for documentation label (open-telemetry#27628)

We see a lot of issues opened that are related to collector usability.
As shared in open-telemetry#27546, documentation can often be added to help clear up
confusion, or provide relevant information. We can use the
`documentation` label for this going forward.

Resolves open-telemetry#27456

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Co-authored-by: bryan-aguilar <46550959+bryan-aguilar@users.noreply.github.com>

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 new or improving existing documentation before closing issues 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.                                                                                                               |