Add set variable function and update docs #2000

Job	Run time
compile_and_lint	1m 10s
test (postgres, postgres, postgres://root:[email protected]/sqlpage)	1m 42s
test (mysql, mysql, mysql://root:[email protected]/sqlpage)	1m 27s
docker_build (linux/amd64)	5m 12s
test (odbc, postgres, Driver=PostgreSQL Unicode;Server=127.0.0.1;Port=5432;Database=sqlpage;UID=r...	1m 42s
docker_build (linux/arm/v7)	5m 30s
test (mssql, mssql, mssql://root:[email protected]/sqlpage)	1m 40s
docker_build (linux/arm64)	5m 36s
windows_test	2m 29s
docker_push	0s
	26m 28s

Provide feedback