chore(kb/ec2): promote ec2 placeholder to article with child

knowledge base/cloud computing/aws/ec2.md

@@ -0,0 +1,57 @@
+# Title
+
+1. [TL;DR](#tldr)
+1. [Disks](#disks)
+1. [Further readings](#further-readings)
+   1. [Sources](#sources)
+
+## TL;DR
+
+<details>
+  <summary>Real world use cases</summary>
+
+```sh
+# Get the IDs of running nginx instances in 'dev'.
+aws ec2 describe-instances --output 'text' \
+  --query 'Reservations[].Instances[].InstanceId[]'
+  --filters \
+    'Name=instance-state-name,Values=running' \
+    'Name=tag:env,Values=dev' \
+    'Name=tag:app,Values=nginx' \
+
+# Start SSM sessions to specific machines.
+aws ec2 describe-instances --output text \
+  --query 'Reservations[].Instances[].InstanceId' \
+  --filters \
+    'Name=app,Values=mysql' \
+    'Name=instance-state-name,Values=running' \
+| xargs -ot aws ssm start-session --target
+```
+
+</details>
+
+## Disks
+
+See [EBS].
+
+## Further readings
+
+- [AWS EC2 Instance pricing comparison]
+- [EC2Instances.info on vantage.sh]
+
+### Sources
+
+<!--
+  References
+  -->
+
+<!-- In-article sections -->
+<!-- Knowledge base -->
+[ebs]: ebs.md
+[ssm]: ssm.md
+
+<!-- Files -->
+<!-- Upstream -->
+<!-- Others -->
+[aws ec2 instance pricing comparison]: https://ec2instances.github.io/
+[ec2instances.info on vantage.sh]: https://instances.vantage.sh/