Add a "What is an extension?" section to Writing guide

[nmcl]

https://quarkus.io/guides/extension-authors-guide

I think a brief section at the start to remind people what an extension is and does, would help the flow.

[nmcl]

Some of this is covered to a degree in "1.1. Why an extension framework" but seems at odds with the section title. Maybe rename that to "What is an extension?" before tackling why we need a framework.

[maxandersen]

related/duplicates quarkusio/quarkus#37288