Add yamllint to linters

We've got a lot of YAML in this repo... as this repo gets more popular, we'll benefit from having some consistency / linter in place.

.github/workflows/ci.yml

@@ -247,3 +247,5 @@ jobs:
           cache: pip
       - run: pip install -r requirements.txt
       - run: ./bin/lint
+      # yamllint is installed in GitHub Actions base runner image: https://github.com/adrienverge/yamllint/pull/588
+      - run: yamllint .

.yamllint.yaml

@@ -0,0 +1,12 @@
+---
+extends: default
+
+rules:
+  # Ignore the line length rule in this repo. 120 chars should be enough for the
+  # stuff we control, but we can't control the test outputs, which are currently
+  # saved in YAML files.
+  line-length:
+    disable
+    # We could downgrade the line length error to a warning, but for now it clutters the output too much.
+    # max: 120
+    # level: warning