Fix images and text on "Stress testing" guide (#1056)

Some of the images on the "Stress testing" guide[1] were removed and
some text was mangled in #978.

[1]: https://k6.io/docs/test-types/stress-testing/

Author: Ivan Mirić

src/data/markdown/translated-guides/en/05 Test Types/03 Stress testing.md

@@ -93,8 +93,9 @@ export default function () {
 </CodeGroup>
 
 The VU chart of a stress test should look similar to this:
-5 minutes. We have also included a recovery stage at the end, where the system is gradually
-decreasing the load to 0.")
+![Virtual user chart of an API stress test](./images/stress-test.png)
+
+This configuration increases the load by 100 users every 2 minutes and stays at this level for 5 minutes. We have also included a recovery stage at the end, where the system is gradually decreasing the load to 0.
 
 If your infrastructure is configured to auto-scale, this test will help you to determine:
 
@@ -205,6 +206,10 @@ export default function () {
 
 The VU chart of a spike test should look similar to this:
 
+![Virtual user chart of a spike test](./images/spike-test.png)
+
+Note, the test starts with a period of 1 minute of low load, a quick spike to very high load, followed by a recovery period of low load.
+
 Remember that the point of this test is to suddenly overwhelm the system. Don't be afraid to increase the number of VUs beyond your worst-case prediction.
 Depending on your needs, you may want to extend the recovery stage to 10 or more minutes to see when the system finally recovers.