docs: use doctest to test README examples

[nicrie]

Instead of using simple code snippets in README use doctest to ensure that shown examples are actually working (part of #130)

[codecov-commenter]

Codecov Report

Attention: 66 lines in your changes are missing coverage. Please review.

Comparison is base (0f95d04) 94.21% compared to head (275dd45) 94.39%.
Report is 61 commits behind head on main.

Files	Patch %	Lines
xeofs/utils/numba_utils.py	20.96%	49 Missing ⚠️
xeofs/data_container/data_container.py	88.88%	3 Missing ⚠️
xeofs/preprocessing/stacker.py	72.72%	3 Missing ⚠️
xeofs/models/_base_cross_model.py	96.07%	2 Missing ⚠️
xeofs/models/_base_model.py	96.07%	2 Missing ⚠️
xeofs/models/decomposer.py	86.66%	2 Missing ⚠️
xeofs/preprocessing/preprocessor.py	88.88%	2 Missing ⚠️
xeofs/utils/xarray_utils.py	88.88%	2 Missing ⚠️
xeofs/utils/io.py	97.95%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #135      +/-   ##
==========================================
+ Coverage   94.21%   94.39%   +0.17%     
==========================================
  Files          71       73       +2     
  Lines        5823     6152     +329     
==========================================
+ Hits         5486     5807     +321     
- Misses        337      345       +8     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.