Expand oracle developer docs

[emmanuel-awosika]

Revamps the page on oracles to provide a more comprehensive overview of oracles and how they work for smart contracts.

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[emmanuel-awosika]

A couple of notes concerning the proposed changes:

• The original page had many parts dedicated to explaining Chainlink's oracle service. Given that there are other oracle services in the ecosystem, I don't think this is particularly fair to other projects. I've tried to reduce this disparity by removing some references to Chainlink, including code snippets. Perhaps we can include those code snippets in a separate tutorial page on how to use the Chainlink services mentioned in the original page (e.g., VRF and Keepers).

• The tutorial at the end of the original page looks like it'll be better off as a separate developer tutorial, so I took that out as well. I've saved the content in a separate doc, though, so I could open a new PR for a "building an oracle tutorial" with the code snippets.

[minimalsm]

Thanks @emmanuel-awosika 😎