DOC: Migrate Read the Docs settings to file and switch from python 3.7 to 3.9

This commit ensures the python version used to build the documentation
matches the one available in Slicer.

See https://docs.readthedocs.io/en/stable/config-file/index.html

Author: jcfr

.gitignore

@@ -5,6 +5,7 @@
 !.gitattributes
 !.git-blame-ignore-revs
 !.pre-commit-config.yaml
+!.readthedocs.yaml
 
 # Ignore all back-up files...
 *~

.readthedocs.yaml

@@ -0,0 +1,21 @@
+# Read the Docs configuration file
+# See https://docs.readthedocs.io/en/stable/config-file/v2.html for details
+
+version: 2
+
+build:
+  os: ubuntu-22.04
+  tools:
+    python: "3.9"
+
+sphinx:
+  builder: html
+  configuration: Docs/conf.py
+
+formats:
+  - epub
+  - pdf
+
+python:
+  install:
+  - requirements: requirements-docs.txt