Add set variable function and update docs #2005

Job	Run time
windows_test	2m 44s
test (odbc, postgres, Driver=PostgreSQL Unicode;Server=127.0.0.1;Port=5432;Database=sqlpage;UID=r...	1m 55s
test (mssql, mssql, mssql://root:[email protected]/sqlpage)	1m 41s
test (postgres, postgres, postgres://root:[email protected]/sqlpage)	1m 57s
compile_and_lint	29s
docker_build (linux/amd64)	4m 53s
docker_build (linux/arm/v7)	5m 2s
docker_build (linux/arm64)	5m 29s
test (mysql, mysql, mysql://root:[email protected]/sqlpage)	1m 16s
docker_push	-1s
	25m 25s

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