docs improvements

[CarloLucibello]

This PR will

• add the missing api references to GNN.jl docs
• transfer the tutorial into the GNN.jl docs (fix [docs] move tutorials to GraphNeuralNetworks.jl package #533)
• remove the global searchbar from the docpage navbar
• collapse by default the docstrings and use ## sections instead of ### and remove index
• filter out heterograph from gnn graph references. Maybe we can use the filter option in autodo https://documenter.juliadocs.org/stable/man/syntax/#@autodocs-block. Use autodoc canonical option also.
• fix some failing doctests