docs: Refactor pausable for erc721 example

[0xNeshi]

Resolves #212

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[bidzyyys]

I am against those changes as they are NOT consistent with our examples.
If we want to merge this PR, also all the rust docs should be updated where needed.

@qalisander @ggonzalez94 wdyt?

[qalisander]

@0xNeshi Pausable doesn't work this way alas. In this case safe_transfer_from_with_data function will call nonoverriden _update function beneath. You can check how example is implemented currently for Pausable extension.
So we should keep when_not_paused check for every public function that calls _update.

Also It's been attempt before to implement Pausable with different approach. But it involves more code duplication..

[qalisander]

I see what I missed. Technically you're calling when_not_paused every time we need it. I think we should refactor our pausable implementation (like here) before updating docs

[0xNeshi]

@qalisander @bidzyyys ahh, life without multi-inheritance is such a pain 😔

The more I look at this code, the more I like the original design. If a dev wants to make a pausable ERC721, they should inherit both erc721 and Pausable, and reimplement all the functions they want to make pausable to just call when_not_paused before calling the relevant erc721 function after it. Although it's a bit boring to have to do that, I think it's much worse to have to actually reimplement all the underlying erc721 function because it all depends on fn _update. It results in much more code duplication.
(Technically speaking, it would have been more correct if I had reimplemented all the erc721 functions I wanted to make pausable and public instead of going around them and calling _update directly. It looks a bit hacky imho)

Maybe we just close this PR and leave things as-is until multi-inheritance is properly supported, what do you guys think?