add .readthedocs.yaml

otherwise rtd uses:
- python 3.7, which is too old for borg.
- shallow git checkout, which might make setuptools_scm fail to create a correct version

.readthedocs.yaml

@@ -0,0 +1,33 @@
+# .readthedocs.yaml - 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: "3.11"
+    jobs:
+        post_checkout:
+            - git fetch --unshallow
+    apt_packages:
+        - build-essential
+        - pkg-config
+        - libacl1-dev
+        - libssl-dev
+        - liblz4-dev
+        - libzstd-dev
+        - libxxhash-dev
+
+python:
+    install:
+        - requirements: requirements.d/development.lock.txt
+        - requirements: requirements.d/docs.txt
+        - method: pip
+          path: .
+
+sphinx:
+    configuration: docs/conf.py
+
+formats:
+    - pdf