Added source Gästrike Återvinnare (#1667)

* Added source Gastrike Återvinnare

* reformat + raise exception instead of returning [] + fixed typo

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Co-authored-by: 5ila5 <5ila5@users.noreply.github.com>

doc/source/gastrikeatervinnare_se.md

@@ -0,0 +1,39 @@
+# Gästrike Återvinnare
+
+Support for schedules provided by [Gästrike Återvinnare](https://gastrikeatervinnare.se/privat/hamtningsdag/), serving the region of Gästrikland, Sweden.
+
+## Configuration via configuration.yaml
+
+```yaml
+waste_collection_schedule:
+  sources:
+    - name: gastrikeatervinnare_se
+      args:
+        street: STREET_NAME
+        city: CITY_NAME
+```
+
+### Configuration Variables
+
+**street**
+*(string) (required)*
+
+**city**
+*(string) (required)*
+
+## Example
+
+```yaml
+waste_collection_schedule:
+  sources:
+    - name: tekniskaverken_se
+      args:
+        street: Bryggargatan 6
+        city: Sandviken
+```
+
+## How to get the source argument
+
+The source argument is the street including number and the city to the house with waste collection.
+The address can be tested [here](https://gastrikeatervinnare.se/privat/hamtningsdag/).
+Note that the city can't be used in the search above, and is only used as a differentiator in case several citys have the same street address.