Add CMake target to generate images for the docs

[maxrjones]

Description of the desired feature

As a follow-up to #6267, we should add a CMake target to generate the images required for building the documentation, so that those without dvc can still build the documentation.

Are you willing to help implement and maintain this feature? Yes

[maxrjones]

I'm running into a problem with example 51 because the script cannot be run without downloading a ~630 MB file. Workarounds include:

1. modify the script to generate an image without needing that file
2. use a GitHub hosted image similar to example 53
3. just require dvc for building the docs and abandon the effort for an alternate option

The downside to the GitHub hosted images is that they are not available in bundled or offline documentation (#6032). But I still prefer option 2 because the example is useful and providing an alternative to dvc is important for allowing people to build the docs from GMT distributions. Any thoughts @GenericMappingTools/gmt-maintainers?

[maxrjones]

Superseded by #6533