Added docs for singleton best practices and exclude_delete

[melinath]

Added docs for singleton resources and exclude_delete.

Fixed hashicorp/terraform-provider-google#20486
Fixed hashicorp/terraform-provider-google#20564

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[modular-magician]

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[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes:

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Errors

google provider:

• Failed to clone repo at new branch
• Failed to clone repo at old branch

google-beta provider:

• Failed to clone repo at new branch
• Failed to clone repo at old branch

terraform-google-conversion:

• Failed to clone repo at new branch
• Failed to clone repo at old branch

Open in Cloud Shell:

• Failed to clone repo at new branch
• Failed to clone repo at old branch

Other:

• Failed to fetch PR data

[melinath]

/gcbrun

[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes:

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Your PR hasn't generated any diffs, but I'll let you know if a future commit does.