deps: update dependency sphinx-rtd-theme to v2 (#330)

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This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
| [sphinx-rtd-theme](https://togithub.com/readthedocs/sphinx_rtd_theme)
| `>=1.3.0,<1.4` -> `>=2,<2.1` |
[![age](https://developer.mend.io/api/mc/badges/age/pypi/sphinx-rtd-theme/2.0.0?slim=true)](https://docs.renovatebot.com/merge-confidence/)
|
[![adoption](https://developer.mend.io/api/mc/badges/adoption/pypi/sphinx-rtd-theme/2.0.0?slim=true)](https://docs.renovatebot.com/merge-confidence/)
|
[![passing](https://developer.mend.io/api/mc/badges/compatibility/pypi/sphinx-rtd-theme/1.3.0/2.0.0?slim=true)](https://docs.renovatebot.com/merge-confidence/)
|
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|

---

### Release Notes

<details>
<summary>readthedocs/sphinx_rtd_theme (sphinx-rtd-theme)</summary>

###
[`v2.0.0`](https://togithub.com/readthedocs/sphinx_rtd_theme/compare/1.3.0...2.0.0)

[Compare
Source](https://togithub.com/readthedocs/sphinx_rtd_theme/compare/1.3.0...2.0.0)

</details>

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setup.py

@@ -46,7 +46,7 @@
     extras_require={
         "docs": [
             "sphinx>=7.2.2,<7.3",
-            "sphinx-rtd-theme>=1.3.0,<1.4",
+            "sphinx-rtd-theme>=2,<2.1",
             "myst-parser>=2.0.0,<2.1",
             "watchdog>=3.0.0,<3.1",
         ],