Gerrit: Added location docs

Documentation have been added that describes how to configure Gerrit
integrations.

Signed-off-by: Niklas Aronsson <niklasar@axis.com>

docs/integrations/gerrit/locations.md

@@ -0,0 +1,43 @@
+---
+id: locations
+title: Gerrit Locations
+sidebar_label: Locations
+description: Integrating source code stored in Gerrit into the Backstage catalog
+---
+
+The Gerrit integration supports loading catalog entities from Gerrit hosted gits. Entities can
+be added to [static catalog configuration](../../features/software-catalog/configuration.md),
+or registered with the
+[catalog-import](https://github.com/backstage/backstage/tree/master/plugins/catalog-import)
+plugin.
+
+## Configuration
+
+To use this integration, add at least one Gerrit configuration to your root `app-config.yaml`:
+
+```yaml
+integrations:
+  gerrit:
+    - host: gerrit.company.com
+      apiBaseUrl: gerrit.company.com/gerrit
+      gitilesBaseUrl: gerrit.company.com/gitiles
+      username: ${GERRIT_USERNAME}
+      password: ${GERRIT_PASSWORD}
+```
+
+Directly under the `gerrit` key is a list of provider configurations, where
+you can list the Gerrit instances you want to fetch data from. Each entry is
+a structure with up to four elements:
+
+- `host`: The host of the Gerrit instance, e.g. `gerrit.company.com`.
+- `apiBaseUrl` (optional): Needed if the Gerrit instance is not reachable at
+  the base of the `host` option (e.g. `https://gerrit.company.com`) set the
+  address here. This is the address that you would open in a browser.
+- `gitilesBaseUrl` (optional): This is needed for creating a valid user-friendly url
+  that can be used for browsing the content of the provider. If not set a default
+  value will be created in the same way as the "baseUrl" option. There is no
+  requirement to have Gitiles for the Backstage Gerrit integration but without it
+  some links in the Backstage UI will be broken.
+- `username` (optional): The Gerrit username to use in API requests. If
+  neither a username nor password are supplied, anonymous access will be used.
+- `password` (optional): The password or http token for the Gerrit user.