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

We see a lot of issues opened that are related to collector usability.
As shared in #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 #27456

---------

Co-authored-by: bryan-aguilar <[email protected]>

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. |