Add set variable function and update docs #2009

Job	Run time
windows_test	2m 36s
test (postgres, postgres, postgres://root:[email protected]/sqlpage)	1m 53s
test (odbc, postgres, Driver=PostgreSQL Unicode;Server=127.0.0.1;Port=5432;Database=sqlpage;UID=r...	1m 51s
compile_and_lint	1m 47s
test (mysql, mysql, mysql://root:[email protected]/sqlpage)	1m 42s
docker_build (linux/arm/v7)	4m 44s
test (mssql, mssql, mssql://root:[email protected]/sqlpage)	1m 34s
docker_build (linux/arm64)	5m 18s
docker_build (linux/amd64)	4m 57s
docker_push	0s
	26m 22s

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