[DOCS][ESQL] Render locate function docs (#107305)

docs/reference/esql/functions/string-functions.asciidoc

@@ -11,6 +11,7 @@
 * <<esql-concat>>
 * <<esql-left>>
 * <<esql-length>>
+* <<esql-locate>>
 * <<esql-ltrim>>
 * <<esql-replace>>
 * <<esql-right>>
@@ -25,6 +26,7 @@
 include::concat.asciidoc[]
 include::layout/left.asciidoc[]
 include::length.asciidoc[]
+include::layout/locate.asciidoc[]
 include::ltrim.asciidoc[]
 include::replace.asciidoc[]
 include::right.asciidoc[]