Add an add-on, cover ZyteLogFormatter in the docs

[Gallaecio]

Follow-up to #126 (comment), with code from #124.

[codecov]

Codecov Report

Attention: Patch coverage is 44.44444% with 15 lines in your changes missing coverage. Please review.

Please upload report for BASE (main@4350d9b). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
zyte_common_items/_addon.py	34.78%	15 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #127   +/-   ##
=======================================
  Coverage        ?   96.83%           
=======================================
  Files           ?       65           
  Lines           ?     2400           
  Branches        ?        0           
=======================================
  Hits            ?     2324           
  Misses          ?       76           
  Partials        ?        0           

Files with missing lines	Coverage Δ
zyte_common_items/__init__.py	100.00% <100.00%> (ø)
zyte_common_items/log_formatters.py	60.00% <ø> (ø)
zyte_common_items/_addon.py	34.78% <34.78%> (ø)