fix(rules): Use true/false, not yes/no

Although `yes` and `no` are recognized as booleans by the pyyaml parser,
the correct keywords are `true` and `false` (as highlighted by the newly
added `truthy` rule).

This commit replaces the use of `yes`/`no` by `true`/`false` and
advertise it in the docs, but also makes sure this change is
backward-compatible (so that `yes` and `no` still work).

yamllint/conf/relaxed.yaml

@@ -25,5 +25,5 @@ rules:
     indent-sequences: consistent
   line-length:
     level: warning
-    allow-non-breakable-inline-mappings: yes
+    allow-non-breakable-inline-mappings: true
   truthy: disable