loki: Add CODEOWNERS file (grafana#9961)

**What this PR does / why we need it**:
This is to have CODEOWNERS file consistent across all projects under
github.com/grafana

**Which issue(s) this PR fixes**:
Fixes #NA

**Special notes for your reviewer**:

**Checklist**
- [x] Reviewed the
[`CONTRIBUTING.md`](https://github.com/grafana/loki/blob/main/CONTRIBUTING.md)
guide (**required**)

Signed-off-by: Kaviraj <kavirajkanagaraj@gmail.com>

CODEOWNERS

@@ -0,0 +1,8 @@
+# Default owners for everything in the repo, unless a later match takes precedence.
+* @grafana/loki-team
+
+# Documentation.
+/docs/ @grafana/docs-logs @grafana/loki-team
+
+# No owners - allows sub-maintainers to merge changes.
+CHANGELOG.md