docs: move doc dependencies from docs/requirements.txt to pyproject.toml

.readthedocs.yaml

 version: 2
 
 build:
-  os: ubuntu-22.04
+  os: ubuntu-24.04
   tools:
-    python: "3.11"
+    python: "3.12"
 
 sphinx:
   configuration: docs/source/conf.py
@@ -12,4 +12,5 @@ python:
   install:
     - method: pip
       path: .
-    - requirements: docs/requirements.txt
+      extra_requirements:
+        - docs

docs/requirements.txt

-sphinx==7.2.5
-sphinx_rtd_theme==1.3.0

docs/source/development/index.rst

@@ -94,7 +94,7 @@ To build the documentation locally for testing:
 
 .. code:: bash
 
-    $ pip install -r docs/requirements.txt
+    $ pip install -e '.[docs]'
     $ cd docs
     $ make html
 

pyproject.toml

@@ -31,6 +31,10 @@ dependencies = [
 ]
 
 [project.optional-dependencies]
+docs = [
+    "sphinx",
+    "sphinx-rtd-theme",
+]
 testing = [
     "pytest",
     "pytest-cov",