Generated docs are sometimes overridden when running full build

[machi1990]

Describe the bug
When running a full build, we sometimes override a previous part of a generated doc

Expected behavior
We should merge extensions doc and not overriding a partial or fully generated one

Actual behavior
Docs are overridden - e.g when running a full build, the quarkus-vertx-http.adoc generated doc does not contain all the information about http configuration.

But when build the vertx-http extension alone, the file in question contains all the information.

Environment (please complete the following information):

• Quarkus version or git rev: problem observed in latest master 9ed6438

Additional context
I'll come up with a fix, still investigating on ways to work around this.