Add a new envsubst operator to replace environment variables in strings (#1082)

pkg/yqlib/doc/operators/headers/envsubst.md

@@ -0,0 +1,10 @@
+# Envsubst
+
+This operator is used to replace environment variables in strings using [envsubst](https://github.com/a8m/envsubst).
+
+To replace environment variables across all values in a document, this can be used with the recursive descent operator
+as follows:
+
+```bash
+yq eval '(.. | select(tag == "!!str")) |= envsubst' file.yaml
+```