readthedocs: Add a configuration file.

This overrides the settings in the web interface, and enables much
more flexible configuration of the build environment (which previously
defaulted to Python 3.7).

https://docs.readthedocs.io/en/stable/config-file/v2.html

Signed-off-by: Anders Kaseorg <anders@zulip.com>

.readthedocs.yaml

@@ -0,0 +1,15 @@
+# https://docs.readthedocs.io/en/stable/config-file/v2.html
+version: 2
+
+build:
+  os: ubuntu-22.04
+  tools:
+    python: "3.10"
+
+sphinx:
+  configuration: docs/conf.py
+  fail_on_warning: true
+
+python:
+  install:
+    - requirements: requirements/docs.txt

docs/requirements.readthedocs.txt

@@ -1 +0,0 @@
-../requirements/docs.txt