Update the Promtail migration topic

[clayton-cornell]

The Promtail migration topic doesn't link to teh Alloy syntax description or reference the default filename extension.

• A high level statement about the syntax: https://grafana.com/docs/alloy/latest/get-started/#configuration-syntax
• A detailed dive into the syntax and how it works: https://grafana.com/docs/alloy/latest/get-started/
• And, the default file name extension: https://grafana.com/docs/alloy/latest/get-started/#configuration-files

Check the other migration topics to see if they could benefit from similar links and def to defaults.

Tip

_{React with 👍 if this issue is important to you.}