chore(docs): Combine Resources and Providers pages

[laurapacilio]

This PR:

• Combines the Resources and Provider pages, based on a suggestion from @danieldreier . They are incredibly related, so even though they aren't set up this way in the Terraform documentation, this combination feels right
• Written contextual information to help users understand what providers and resources are and how they use them in their applications.
• Reordered original content from docs to help with flow
• Done an initial rough copyedit
• Added some TODOs for Engineering.

[schersh]

I will leave the technical review for engineering, but just added a few suggested copy edits. Thanks!

[laurapacilio]

awesome! @danieldreier and/or @skorfmann let us know what you think! And if you have input for the TODOs please put it in comments/suggestions and I can add it in!

[github-actions]

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