readthedocs: Add the configuration file.

Since last month ReadTheDocs only supports building with a new
configuration file provided in the repository itself:
  https://blog.readthedocs.com/migrate-configuration-v2/

So, all our documentation builds are failing for quite some time.

Add the configuration file to unblock documentation updates.

Signed-off-by: Ilya Maximets <i.maximets@ovn.org>

.readthedocs.yaml

@@ -0,0 +1,26 @@
+# .readthedocs.yaml
+# Read the Docs configuration file.
+# See https://docs.readthedocs.io/en/stable/config-file/v2.html for details.
+
+# Required.
+version: 2
+
+# Set the OS, Python version, etc.
+build:
+  os: ubuntu-22.04
+  tools:
+    python: "3.12"
+
+# Build documentation in the "Documentation/" directory with Sphinx.
+sphinx:
+  configuration: Documentation/conf.py
+  # Default HTML builder.
+  builder: "html"
+
+# Build all formats: HTML, PDF, ePub.
+formats: all
+
+# Declare the Python requirements.
+python:
+  install:
+  - requirements: Documentation/requirements.txt

Makefile.am

@@ -100,6 +100,7 @@ EXTRA_DIST = \
 	.github/workflows/test.yml \
 	.github/workflows/ovn-kubernetes.yml \
 	.github/workflows/ovn-fake-multinode-tests.yml \
+	.readthedocs.yaml \
 	boot.sh \
 	$(MAN_FRAGMENTS) \
 	$(MAN_ROOTS) \