Refactor pyproject.toml: move docs dependencies

- Moved `limix-sphinx-theme`, `sphinx`, and `sphinx-rtd-theme` to extras in `pyproject.toml`

This change optimizes dependency management by grouping documentation dependencies
under a single optional `docs` extra, streamlining the build configuration. 📚✨

pyproject.toml

@@ -28,11 +28,8 @@ requests = "^2.32.3"
 [tool.poetry.group.dev.dependencies]
 pytest = "^8.3.3"
 
-
-[tool.poetry.group.docs.dependencies]
-limix-sphinx-theme = "^0.0.4"
-sphinx = "^8.1.3"
-sphinx-rtd-theme = "^3.0.1"
+[tool.poetry.extras]
+docs = ["limix-sphinx-theme", "sphinx", "sphinx-rtd-theme"]
 
 [build-system]
 requires = ["poetry-core"]