add document for system variable tidb_read_consistency (#9897)
add document for system variable tidb_read_consistency (#9897)

* Update system-variables.md

* Update system-variables.md

* Update system-variables.md

* Update system-variables.md

Co-authored-by: you06 <d046ed4ccb48113a1cfb6836902d451889cc779c@gmail.com>

* Update system-variables.md

* Update system-variables.md

Co-authored-by: TomShawn <3d0966147538bc5ab020e8dffa2c0b746f9728a6@users.noreply.github.com>

* Update system-variables.md

Co-authored-by: TomShawn <3d0966147538bc5ab020e8dffa2c0b746f9728a6@users.noreply.github.com>

* fix ci

* reorder

Co-authored-by: you06 <d046ed4ccb48113a1cfb6836902d451889cc779c@gmail.com>
Co-authored-by: TomShawn <3d0966147538bc5ab020e8dffa2c0b746f9728a6@users.noreply.github.com>
pingcap/docs,pingcap/docs,pingcap/docs