[DOC] Building docs in CI

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Description

Added sphinx_design to the documentation requirements. This package provides additional directives and roles for Sphinx documentation, enhancing the layout and design capabilities for our documentation.

Relates to #N/A

Checklist

• My code uses type hinting for function and method arguments and return values.
• I have created tests which cover my code.
• The test code either 1. demonstrates at least one valuable use case (e.g. integration tests) or 2. verifies that outputs are as expected for given inputs (e.g. unit tests).
• New tests pass locally with my changes.

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• [DOC] Building docs in CI #999 👈 (View in Graphite)
• main

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I need to get the gempy-le sdk running before finishing this one