Document how to install Loki and Promtail using APT/DNF. (#8841)

**What this PR does / why we need it**:
With #6456 we started providing native packages of Loki, Promtail and
logcli. They've been added to Grafana's repository in the meantime.

In order to spread the knowledge of the packages and the repo it should
be documented.

**Which issue(s) this PR fixes**:
Fixes #52


**Checklist**
- [ ] Reviewed the
[`CONTRIBUTING.md`](https://github.com/grafana/loki/blob/main/CONTRIBUTING.md)
guide (**required**)
- [x] Documentation added
- [ ] Tests updated
- [ ] `CHANGELOG.md` updated
- [ ] Changes that require user attention or interaction to upgrade are
documented in `docs/sources/upgrading/_index.md`

---------

Co-authored-by: René Scheibe <rene.scheibe@gmail.com>
Co-authored-by: J Stickler <julie.stickler@grafana.com>

docs/sources/installation/local.md

@@ -11,8 +11,23 @@ In order to log events with Grafana Loki, download and install both Promtail and
 
 The configuration specifies running Loki as a single binary.
 
-## Install
+## Install using APT or RPM package manager
 
+1. Add Granafa's Advanced Package Tool [APT](https://apt.grafana.com/) or RPM Package Manager [RPM](https://rpm.grafana.com/)
+   package repository following the linked instructions.
+1. Install Loki and Promtail
+   1. Using `dnf`
+      ```
+      dnf update
+      dnf install loki promtail
+      ```
+   1. Using `apt-get`
+      ```
+      apt-get update
+      apt-get install loki promtail
+      ```
+
+## Install manually
 1. Navigate to the [release page](https://github.com/grafana/loki/releases/).
 2. Scroll down to the Assets section under the version that you want to install.
 3. Download the Loki and Promtail .zip files that correspond to your system.