SQL: documentation improvements and updates (#36918)

* Added Limitations page
* Made the aggregations page follow the common template for functions
* Modified all tables to have the first row's cells content centered
* Polishing in other various sections

Author: astefan

docs/reference/sql/appendix/syntax-reserved.asciidoc

@@ -16,11 +16,13 @@ be quoted (using double quotes) in order to be used as an identifier, for exampl
 SELECT "AS" FROM index
 ----
 
-[cols="^,^,^",options="header"]
+[cols="^,^,^"]
 
 |===
 
-|Keyword                      |SQL:2016      |SQL-92
+s|Keyword
+s|SQL:2016
+s|SQL-92
 
 
 |`ALL`                        |reserved      |reserved

docs/reference/sql/concepts.asciidoc

@@ -15,6 +15,8 @@ Last but not least, {es-sql} tries to obey the https://en.wikipedia.org/wiki/Pri
 
 === Mapping concepts across SQL and {es}
 
+beta[]
+
 While SQL and {es} have different terms for the way the data is organized (and different semantics), essentially their purpose is the same.
 
 So let's start from the bottom; these roughly are:

docs/reference/sql/endpoints/client-apps/squirrel.asciidoc

@@ -1,13 +1,13 @@
 [role="xpack"]
 [testenv="platinum"]
 [[sql-client-apps-squirrel]]
-=== SQquirelL SQL
+=== SQuirreL SQL
 
 beta[]
 
 [quote, http://squirrel-sql.sourceforge.net/]
 ____
-http://squirrel-sql.sourceforge.net/[SQuirelL SQL] is a graphical, [multi-platform] Java program that will allow you to view the structure of a JDBC compliant database [...].
+http://squirrel-sql.sourceforge.net/[SQuirreL SQL] is a graphical, [multi-platform] Java program that will allow you to view the structure of a JDBC compliant database [...].
 ____
 
 ==== Prerequisites

docs/reference/sql/endpoints/jdbc.asciidoc

@@ -138,6 +138,8 @@ Opens up a {es-sql} connection to `server` on port `3456`, setting the JDBC conn
 
 === API usage
 
+beta[]
+
 One can use JDBC through the official `java.sql` and `javax.sql` packages:
 
 ==== `java.sql`