fix(plugin-nested-docs): custom overrides of breadcrumb and parent fields

[DanRibbens]

Description

fix #3692

1. Overrides were not spread in the correct order in createBreadcrumbsField
2. You can now use the create field functions without also needing to specify the breadcrumbsFieldSlug or parentFieldSlug
3. breadcrumbsFieldSlug was not being used in the parent field filterOptions, resaveSelfAfterCreate or resaveChildren hooks
4. parentFieldSlug was not being used in resaveChildren
5. custom parentFieldSlug was not being used in getParents

• [x] I have read and understand the CONTRIBUTING.md document in this repository.

Type of change

• [x] Bug fix (non-breaking change which fixes an issue)
• [x] This change requires a documentation update

Checklist:

• [x] I have added tests that prove my fix is effective or that my feature works
• [x] Existing test suite passes locally with my changes
• [x] I have made corresponding changes to the documentation