[Documentation] Typos & link fix

[SimonPop]

Hi everyone!

I was reading through the introduction tutorial of the library and could spot minor typos. I decided to try and fix some of them.

At the same time, I updated the internal links to classes and methods when I could.

Here is the list of modifications proposed in this commit:

• Correcting minor typos.
• Updating internal links when the path is no longer correct (e.g. torch_frame.TensorFrame -> torch_frame.data.TensorFrame).
• Including the 's' for plural links into the pastil using a trick described in https://stackoverflow.com/questions/19403972/writing-terms-in-plural-without-the-redundancy (e.g. :class:torch.Tensor\ s -> :class:Tensors<torch.Tensor>).
• Providing links for existing classes previously unlinked (e.g. :obj:Dataset -> :obj:torch_frame.data.Dataset).

I don't know if tooling exists to maintain documentation when refactoring, but hopefully, this commit can refresh it slightly for now.

Do tell me if you think I have missed something that could be updated in this same commit, or if I did change something that was intended to be as it was.

Thank you for creating this library, I am very enthusiastic about it!

[weihua916]

Thank you so much! LGTM.