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.

Need to remove the upper restriction on the sphinx version.
sphinx 2.0 is very old at this point and pip fails to install
it along with other dependencies on the rtd server.

Note: Sphinx 2.0 moved from HTML4 to HTML5 renderer and tables
no longer have borders by default.  That should be addressed
via CSS file in the ovs-sphinx-theme.

Acked-by: Aaron Conole <aconole@redhat.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>

.readthedocs.yaml

@@ -0,0 +1,24 @@
+# .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
+
+# Build all formats: HTML, PDF, ePub.
+formats: all
+
+# Declare the Python requirements.
+python:
+  install:
+  - requirements: Documentation/requirements.txt

Documentation/requirements.txt

@@ -1,2 +1,2 @@
-sphinx>=1.1,<2.0
+sphinx>=1.1
 ovs_sphinx_theme>=1.0,<1.1

Makefile.am

@@ -84,6 +84,7 @@ EXTRA_DIST = \
 	.cirrus.yml \
 	.editorconfig \
 	.github/workflows/build-and-test.yml \
+	.readthedocs.yaml \
 	appveyor.yml \
 	boot.sh \
 	poc/builders/Vagrantfile \