docs(rtd): install docs dependency group in RTD build

.readthedocs.yml

@@ -10,16 +10,19 @@ build:
   os: ubuntu-22.04
   tools:
     python: "3.11"
-  # todo: uncomment below if notebooks are to be executed in RTD build
-  # instead of in CI as currently, requires runtime reduction <15 mins
-  # jobs:
-  #   post_create_environment:
-  #     # install MODFLOW6 and related executables (notebooks need them)
-  #     - get-modflow :python
-  #     # swap default vtk for vtk-osmesa (avoid PyVista openGL errors)
-  #     # https://github.com/pyvista/pyvista/blob/main/.github/workflows/vtk-pre-test.yml#L53
-  #     - pip uninstall -y vtk
-  #     - pip install --extra-index-url https://wheels.vtk.org trame vtk-osmesa
+  jobs:
+    install:
+      - pip install --upgrade "pip>=25.1"
+      - pip install --group docs .
+    # todo: uncomment below if notebooks are to be executed in RTD build
+    # instead of in CI as currently, requires runtime reduction <15 mins
+    # post_install:
+    #   # install MODFLOW6 and related executables (notebooks need them)
+    #   - get-modflow :python
+    #   # swap default vtk for vtk-osmesa (avoid PyVista openGL errors)
+    #   # https://github.com/pyvista/pyvista/blob/main/.github/workflows/vtk-pre-test.yml#L53
+    #   - pip uninstall -y vtk
+    #   - pip install --extra-index-url https://wheels.vtk.org trame vtk-osmesa
 
 # Build documentation in the docs/ directory with Sphinx
 sphinx:
@@ -28,11 +31,3 @@ sphinx:
 # Build only html and htmlzip (skip others, including PDF and EPUB)
 formats:
   - htmlzip
-
-# Set the Python version and requirements
-python:
-  install:
-    - method: pip
-      path: .
-      extra_requirements:
-        - doc