Add ReadTheDocs support

Mirror versioned documentation at https://ceres-solver.readthedocs.io.

Change-Id: I180e8fc32d05a0fa7dce09f77dbe38c5ca207420

.readthedocs.yaml

diff --git a/.readthedocs.yaml b/.readthedocs.yaml
new file mode 100644
index 0000000..8cb632a
--- /dev/null
+++ b/.readthedocs.yaml
@@ -0,0 +1,14 @@
+version: 2
+
+build:
+  os: ubuntu-24.04
+  tools:
+    python: "3.13"
+
+sphinx:
+  configuration: docs/source/conf.py
+  fail_on_warning: true
+
+python:
+  install:
+    - requirements: docs/requirements.txt