Add docs on how to switch backend serialisers.
Add docs on how to switch backend serialisers.

ghislainp/blitzdb,orgkhnargh/blitzdb,kylewm/blitzdb,programmdesign/blitzdb,adewes/blitzdb,programmdesign/blitzdb,quantifiedcode-bot/blitzdb,cwoebker/blitzdb,programmdesign/blitzdb