docs: developer installation

[pabloitu]

docs: Fixed #234 command instruction to install pycsep with pytest and sphinx dependencies

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Fixes issue #(please fill in or delete if not needed).

Type of change:

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• Documentation update (this pull request adds no new code)

Checklist:

• New and existing unit tests pass locally with my changes

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (1e2badb) 54.95% compared to head (942e45b) 54.95%.

❗ Current head 942e45b differs from pull request most recent head f08f903. Consider uploading reports for the commit f08f903 to get more accurate results

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Additional details and impacted files

@@           Coverage Diff           @@
##             main     #246   +/-   ##
=======================================
  Coverage   54.95%   54.95%           
=======================================
  Files          23       23           
  Lines        3963     3963           
  Branches      578      578           
=======================================
  Hits         2178     2178           
  Misses       1648     1648           
  Partials      137      137           

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[fabiolsilva]

Minor documentation update to clarify installation steps.