minor #17724 [ExpressionLanguage] Merge docs about ExpresssionLanguage (fabpot)

This PR was squashed before being merged into the 5.4 branch.

Discussion
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[ExpressionLanguage] Merge docs about ExpresssionLanguage

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3ecb282 [ExpressionLanguage] Merge docs about ExpresssionLanguage

Author: javiereguiluz

_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