[Docs] Add howToSPI to TOC
[Docs] Add howToSPI to TOC

Added documents on how to use SPI API
to the How To section of Legato.io

Resolves: LE-5075
Change-Id: I224b680fad6aaf70117d5983ab4224650e22e396

legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af