Update .readthedocs.yml (flashlight#1085)

Summary:
Enable docs build with PRs on RTD. Update to use the new [build schema](https://docs.readthedocs.io/en/stable/config-file/v2.html#build-os) and deprecate old properties. Python 3.10 (or something somewhat modern) is required for newer versions of Sphinx/Breathe.

Pull Request resolved: flashlight#1085

Test Plan: CI

Reviewed By: richjames0

Differential Revision: D44081047

Pulled By: jacobkahn

fbshipit-source-id: 5867451345de3593ff7c5153e1fdd182286c1678

.readthedocs.yml

@@ -5,12 +5,16 @@
 # Required
 version: 2
 
+build:
+  os: ubuntu-22.04
+  tools:
+    python: "3.11"
+
 # Build documentation with Sphinx
 sphinx:
   configuration: docs/source/conf.py
 
 # Python version and requirements for build
 python:
-  version: 3.7
   install:
     - requirements: docs/requirements.txt