Add set variable function and update docs #2004

Job	Run time
compile_and_lint	1m 11s
test (mysql, mysql, mysql://root:[email protected]/sqlpage)	1m 19s
test (postgres, postgres, postgres://root:[email protected]/sqlpage)	1m 34s
test (odbc, postgres, Driver=PostgreSQL Unicode;Server=127.0.0.1;Port=5432;Database=sqlpage;UID=r...	1m 34s
test (mssql, mssql, mssql://root:[email protected]/sqlpage)	1m 32s
docker_build (linux/amd64)	5m 28s
docker_build (linux/arm/v7)	4m 57s
docker_build (linux/arm64)	5m 34s
windows_test	2m 49s
docker_push	0s
	25m 58s

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