Add example for setting Wyckoff sites

[janbridley]

Description

Code for modifying the wyckoff sites of parsed files has been added, enabling the creation of new structures and the modification of existing ones. While some modifications were possible previously, the use of structured arrays makes adding or removing sites nontrivial. The new functionality aids this, and makes it very clear that modifying position data may invalidate fields from the original structure

Motivation and Context

Types of Changes

• Documentation update
• Bug fix
• New feature
• Breaking change¹

¹The change breaks (or has the potential to break) existing functionality and should be merged into the breaking branch

Checklist:

• I am familiar with the Development Guidelines
• The changes introduced by this pull request are covered by existing or newly introduced tests.
• I have updated the changelog and added my name to the credits.

[janbridley]

As discussed in EBT today, it's fortunate I was working on something that may be helpful. @sumitavaumich please refer to the new example_new_structures doc page to see if it meets the requirements of what you were looking for -- if not, I'm happy to expand on it!

https://parsnip-cif--124.org.readthedocs.build/en/124/example_new_structures.html