Better aiven-extras documentation

[lornajane]

From a feedback comment on https://developer.aiven.io/docs/products/postgresql/concepts/dba-tasks-pg:

"The aiven extra extension repo doesnt really have any documentation and the above doesnt really help either"

Check the article, see if we can add more information about the extension, and also review the project docs if we should add something there instead/as well.