More verbose error message when creating extension

What does this MR do?

This adds a migration helper to create (and drop) a postgres extension. It is meant to provide a more helpful error message in case the extension cannot be created (or dropped) due to missing privileges.

See #243729 (closed) for more background.

This also adds a documentation page to explain this problem in more detail.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Closes #243729 (closed)