[Docs]: Add tutorial/guide for building custom extension images

### Is there an existing issue already for your request/idea?

- [x] I have searched for an existing issue, and could not find anything. I believe this is a new documentation enhancement to be evaluated.

### What problem in the existing documentation this issue aims to solve?

The documentation quite clearly outlines using the new image catalog and where to find the official one with the limited extensions in there. It would be nice to provide some working examples of building extension images with Postgres extensions that do not ship a .deb package. I have just gone through this process, building an extension image for pg_duckdb.

People are always going to want to add additional extensions beyond those officially supported by the project, it would be very handy to have a guide on how to achieve this, along with the common pitfalls, or why you might not want to do this.

### Describe what additions need to be done to the documentation

_No response_

### Describe what pages need to change in the documentation, if any

_No response_

### Describe what pages need to be removed from the documentation, if any

_No response_

### Additional context

_No response_

### Backport?

Yes

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct