[DOCS] Update documentation to use specific packages rather than @microsoft/mgt

[sebastienlevert]

Adding a note. We're still referencing @microsoft/mgt on the root of the repo

• https://github.com/microsoftgraph/microsoft-graph-toolkit#packages --> Should we remove it entirely?
• https://github.com/microsoftgraph/microsoft-graph-toolkit#use-via-npm

[Mnickii]

@sebastienlevert I think we leave it there as a quick testing option, because the rest of the docs guide the user to use specific packages.