DOC: add SPSS comparison guide structure
PR adds a comparison guide for SPSS users transitioning to pandas, addressing #60727. 

## Changes
- Creates new `comparison_with_spss.rst` in getting_started/comparison/
- Documents basic data structure mappings:
  - SPSS data files → pandas DataFrames
  - SPSS variables → pandas Series
  - SPSS case numbers → pandas Index
- Sets up structure for common operations comparison

## Implementation
- Following existing comparison guide formats (R, SQL, etc.)
- Using reStructuredText format
- Including executable code examples
- Cross-referencing pandas documentation

- [ x] closes #60727
Hi, I am not sure why this check is not passing. Could you please provide some insight? Thanks @Jacob-Lazar 