Adding single-example target and facilitating example contribution

[RobPasMue]

I came up with the following guidelines which I am planning to add (somewhere) in the future. Leaving them here as reference in the meantime

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Facilitating contributions in the form of examples

One of the main challenges we face inside our libraries is the lack of examples. Examples are
critical for users to understand how to use the library and present new functionality. They are also
a great mechanism to educate users on the best practices of using it.

When users want to contribute examples, we need to make it as easy as possible for them to do so. This
means that we need to provide a clear structure for them to follow. We also need to make sure that the
build process is as simple as possible.

3 best practices for examples are:

• Providing a template: This is a great way to ensure that users follow the same structure. It also makes it easy for
  them to get started. The template should include all the necessary information, such as the name of the
  example, a description, and the code itself.
• Verifying the example: This is a great way to ensure that the example works as expected. It also helps to catch any
  errors before they are submitted. The verification process should include running the example and checking
  the output. If the example fails, it should be easy for the user to fix it.

Templates

Here an example on how to promote the use of templates for examples:

• PyAnsys Geometry: https://geometry.docs.pyansys.com/version/stable/examples/99_misc/template.html
• PyPrimeMesh: https://prime.docs.pyansys.com/version/stable/examples/gallery_examples/misc/example_template.html#sphx-glr-examples-gallery-examples-misc-example-template-py

These templates are also referred to in the contributing guidelines.

• PyAnsys Geometry: https://geometry.docs.pyansys.com/version/stable/contributing.html#adding-examples

Verifying examples

The verification process will depend on each library and the tools used for generating the examples. Within
our ecosystem, we are using Sphinx Gallery and NBSphinx to generate the examples, and the build process
is slightly different for each library.

However, the following approach could be useful for all libraries:

• Create a Makefile target that only runs a specific example.

This target should be used to run the example and check the output. If the example fails, it should be easy for the user to fix it.
An example of this is the make target in PyAnsys Geometry:

./make.bat single-example examples/01_getting_started/01_math.mystnb

make single-example examples/01_getting_started/01_math.mystnb

The pull request in which this was added is: ansys/pyansys-geometry#1893
This implementation is specific for NB Sphinx, but the idea is to create a target that runs a specific example and checks the output.