[ExpressionLanguage] Merge docs about ExpresssionLanguage

Author: fabpot

_build/redirection_map

@@ -538,3 +538,5 @@
 /components/security /security
 /components/var_dumper/advanced /components/var_dumper#advanced-usage
 /components/yaml/yaml_format /components/yaml#yaml-format
+/components/expression_language/syntax /components/expression_language#expression-language-syntax
+/components/expression_language/extending /components/expression_language#expression-language-extending