docs: add optional Sphinx dependencies for documentation

- Added `limix-sphinx-theme`, `sphinx`, and `sphinx-rtd-theme` to `pyproject.toml`.

These additions enable the generation of enhanced documentation by including
Sphinx themes as optional dependencies. This helps with better maintenance and
readability of project documentation. 📘

pyproject.toml

@@ -23,6 +23,9 @@ texttable = "^1.7.0"
 tqdm = "^4.66.5"
 xarray = "^2024.10.0"
 requests = "^2.32.3"
+limix-sphinx-theme = { version = "^0.0.4", optional = true }
+sphinx = { version = "^8.1.3", optional = true }
+sphinx-rtd-theme = { version = "^3.0.1", optional = true }
 
 [tool.poetry.group.dev.dependencies]
 pytest = "^8.3.3"