A website concists of many aspects, one of which can and usually is a sidebar. You could code one yourself or you could use one off the shelf that has been tried and tested.

https://www.codeply.com/go/8ESO56VMns/bootstrap-4-sidebar-collapse

https://github.com/BlackrockDigital/startbootstrap-simple-sidebar

https://bootstrapious.com/p/bootstrap-sidebar