Add metadata to each page

docs/reference/esql/esql-functions.asciidoc

@@ -1,6 +1,12 @@
 [[esql-functions]]
 == ESQL functions
 
+++++
+<titleabbrev>Functions</titleabbrev>
+++++
+:keywords: {es}, ESQL, {es} query language, functions
+:description: ESQL supports various functions for calculating values. 
+
 <<esql-row,`ROW`>>, <<esql-eval,`EVAL`>> and <<esql-where,`WHERE`>> support
 these functions:
 

docs/reference/esql/esql-processing-commands.asciidoc

@@ -1,5 +1,11 @@
 [[esql-processing-commands]]
-== Processing commands
+== ESQL processing commands
+
+++++
+<titleabbrev>Processing commands</titleabbrev>
+++++
+:keywords: {es}, ESQL, {es} query language, processing commands
+:description: ESQL processing commands change an input table by adding, removing, or changing rows and columns.
 
 ESQL processing commands change an input table by adding, removing, or changing
 rows and columns.

docs/reference/esql/esql-source-commands.asciidoc

@@ -1,5 +1,11 @@
 [[esql-source-commands]]
-== Source commands
+== ESQL source commands
+
+++++
+<titleabbrev>Source commands</titleabbrev>
+++++
+:keywords: {es}, ESQL, {es} query language, source commands
+:description: An ESQL source command produces a table, typically with data from {es}. 
 
 An ESQL source command produces a table, typically with data from {es}. 
 

docs/reference/esql/esql-syntax.asciidoc

@@ -1,5 +1,11 @@
 [[esql-syntax]]
-== Syntax reference
+== ESQL syntax reference
+
+++++
+<titleabbrev>Syntax reference</titleabbrev>
+++++
+:keywords: {es}, ESQL, {es} query language, syntax
+:description: An ESQL query is composed of a source command followed by an optional series of processing commands, separated by a pipe character.
 
 [discrete]
 [[esql-basic-syntax]]