Fix duplicated TOC in writing-extensions.adoc
Fix duplicated TOC in writing-extensions.adoc

Don't ask me why, when having the _attributes.adoc below :toc:, we end
up with a second TOC on the website.

Fixes #28747

quarkusio/quarkus,quarkusio/quarkus,quarkusio/quarkus,quarkusio/quarkus,quarkusio/quarkus