website: Add data consistency error docs for "planned value does not match config value"

[austinvalle]

Ref: #1229

This PR adds additional documentation about a common data consistency error we see during migration to Plugin Framework:

TIMESTAMP [WARN]  Provider "TYPE" produced an invalid plan for ADDRESS, but we are tolerating it because it is using the legacy plugin SDK.
    The following problems may be the cause of any confusing errors from downstream operations:
      - .ATTRIBUTE: planned value cty.StringVal("VALUE") does not match config value cty.StringVal("value")

I added a simplified example for demonstrating the behavior, but can adjust/add more examples if we think that'd be helpful.

[bendbennett]

LGTM 🚀

[bflad]

I don't want to hold this excellent content up from going somewhere, but something to potentially consider here. 👍

[austinvalle]

@bflad @bendbennett - I moved the code over to a new PR in plugin-framework. I removed the migration section + plugin framework code from this PR and added an SDKv2 solution.

I'd love a second review on the SDKv2 solution I provided, as I was basing a lot of it off my understanding of the Semantic Equality RFC and some sandbox testing. Want to make sure this example makes sense 😄

[bflad]

This looks good to me 🚀

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