fix: add yamllint config and remove trailing blank line in pr-build.yml (#80)

Add root .yamllint.yml with relaxed rules matching the megalinter config
and disable the truthy rule that flags `on:` in GitHub Actions workflows.
Remove trailing blank line from pr-build.yml.

.yamllint.yml

@@ -0,0 +1,13 @@
+---
+extends: default
+rules:
+  braces:
+    max-spaces-inside: 1
+  new-lines:
+    level: warning
+    type: unix
+  line-length:
+    max: 500
+  comments:
+    min-spaces-from-content: 1
+  truthy: disable