Fix performance improvements indentation and link

Why is this change being made?

The example given for performance improvements will cause the pipeline to fail if inserted in a performance-improvements.yml file in the unreleased directory. This is because it needs to be nested under features > secondary in Yaml.

Also, the URL 404s.

This MR fixes the indentation, the URL and adds examples for how to add items to this section.

Does this MR meet the acceptance criteria?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what