Refresh doc environment (#273)

* Updates to [RTD config file version 2]( https://docs.readthedocs.io/en/stable/config-file/v2.html#configuration-file-v2 )
* Use Mamba on RTD
* Places doc environment file next to other environment files
* Updates dependencies based on the Python 3.8 environment file + recent Sphinx, etc.

.readthedocs.yml

@@ -0,0 +1,12 @@
+version: 2
+
+build:
+  os: ubuntu-22.04
+  tools:
+    python: "mambaforge-4.10"
+
+sphinx:
+  configuration: docs/conf.py
+
+conda:
+  environment: continuous_integration/environment-doc.yml

CONTRIBUTING.rst

@@ -47,7 +47,7 @@ conda environment for building the dask-image documentation:
 
 .. code-block:: console
 
-    $ conda env create -f environment_doc.yml
+    $ conda env create -f continuous_integration/environment-doc.yml
     $ conda activate dask_image_doc_env
 
 This conda environment contains dask-image and its dependencies, sphinx,

continuous_integration/environment-doc.yml

@@ -0,0 +1,19 @@
+name: dask_image_doc_env
+
+channels:
+  - conda-forge
+
+dependencies:
+  - python=3.8.*
+  - pip==22.3
+  - wheel==0.37.1
+  - sphinx==5.3.0
+  - jinja2==3.1.2
+  - dask==2.8.1
+  - numpy==1.23.4
+  - scipy==1.9.2
+  - scikit-image==0.19.3
+  - pims==0.6.1
+  - slicerator==1.1.0
+  - pip:
+      - dask-sphinx-theme>=3.0.0