GraphNeuralNetworks-v0.6.23

Diff since GraphNeuralNetworks-v0.6.22

Merged pull requests:

• improve GNNLux docs (#542) (@CarloLucibello)
• Add version selector (#543) (@aurorarossi)
• Add Hands On tutorial GNNLux (#549) (@aurorarossi)
• try new approach for building and deploying docs (#552) (@CarloLucibello)

Closed issues:

• Stack Layers in a HeteroGraphConv Model (#541)
• [lux] add tutorials (#544)