Update sources

[khaeru]

• Inherit from abs.Source in abs_json.Source.
  Correct an error introduced in Update ABS with SDMX-ML support #129. Re-using just one method, instead of subclassing, prevented super() from working in the handle_response() hook.
• Use https://validator.w3.org/checklink to check broken links in doc/sources.rst
• Add 4 new sources operated by Eurostat: ESTAT_COMEXT, COMP, EMPL, and GROW.

PR checklist

• Checks all ✅
• Update documentation.
• Update doc/whatsnew.rst.

[codecov]

Codecov Report

Merging #130 (f8d9f66) into main (14f6a01) will decrease coverage by 2.47%.
The diff coverage is 76.00%.

@@            Coverage Diff             @@
##             main     #130      +/-   ##
==========================================
- Coverage   98.30%   95.83%   -2.47%     
==========================================
  Files          73       74       +1     
  Lines        5602     5619      +17     
==========================================
- Hits         5507     5385     -122     
- Misses         95      234     +139     

Impacted Files	Coverage Δ
sdmx/model/common.py	100.00% <ø> (ø)
sdmx/source/__init__.py	100.00% <ø> (ø)
sdmx/source/estat_comext.py	57.14% <57.14%> (ø)
sdmx/tests/test_sources.py	67.74% <81.25%> (-27.00%)	⬇️
sdmx/source/abs_json.py	100.00% <100.00%> (ø)
sdmx/tests/test_source.py	100.00% <100.00%> (ø)

... and 14 files with indirect coverage changes

[khaeru]

The codecov/patch check failure is due to the added sources, and will be covered by the "services" job run on main.