Parse structure information in `Item.description`

[khaeru]

Closes #133.

PR checklist

• [x] Checks all ✅
• [x] Update documentation
• [x] Update doc/whatsnew.rst

[codecov[bot]]

Codecov Report

Merging #137 (01f2268) into main (772caee) will decrease coverage by 2.19%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #137      +/-   ##
==========================================
- Coverage   98.26%   96.08%   -2.19%     
==========================================
  Files          82       85       +3     
  Lines        6334     6406      +72     
==========================================
- Hits         6224     6155      -69     
- Misses        110      251     +141     

Files Changed	Coverage Δ
sdmx/testing/__init__.py	99.28% <ø> (ø)
sdmx/dictlike.py	97.84% <100.00%> (ø)
sdmx/message.py	100.00% <100.00%> (ø)
sdmx/model/common.py	99.89% <100.00%> (+<0.01%)	:arrow_up:
sdmx/model/v21.py	100.00% <100.00%> (ø)
sdmx/tests/test_docs.py	100.00% <100.00%> (ø)
sdmx/tests/test_util.py	100.00% <100.00%> (ø)
sdmx/tests/util/test_item_structure.py	100.00% <100.00%> (ø)
sdmx/util/__init__.py	100.00% <100.00%> (ø)
sdmx/util/item_structure.py	100.00% <100.00%> (ø)
... and 1 more

... and 16 files with indirect coverage changes