.readthedocs.yaml and docs requirements

Add .readthedocs.yaml to fix rtd-builds and add docs requirements
(replacing sphinx_requirements.txt) via extras section in setup.py.

This ignores our Jinja2 version fixation! Let's see if that works!

.readthedocs.yaml

@@ -0,0 +1,16 @@
+version: 2
+
+build:
+  os: ubuntu-22.04
+  tools:
+    python: "3.11"
+
+sphinx:
+  configuration: docs/source/conf.py
+
+python:
+  install:
+    - method: pip
+      path: .
+      extra_requirements:
+        - docs

setup.py

@@ -29,4 +29,11 @@
         packages=[
             'discogs_client',
             ],
-        )
+       extras_require={
+           "docs": [
+               "sphinx",
+               "sphinx-rtd-theme",
+               "myst-parser",
+           ],
+       }
+)