Updating maintainers.md (grafana#6466)

* Updating maintainers.md

To be consistent with other OSS products, adding the maintainer of the technical documentation to the list of maintainers.

* Update MAINTAINERS.md

Adding further detail to properly explain maintainer role for docs.

Author: GrafanaWriter

MAINTAINERS.md

@@ -1 +1,5 @@
 @slim-bean is the main/default maintainer.
+
+Some parts of the codebase have other maintainers:
+- `docs`:
+  - @KMiller-Grafana - [@KMiller-Grafana ](https://github.com/@kmiller-grafana) ([Grafana Labs](https://grafana.com/))