docs: add example of terraform-registry configuration

[xlgmokha]

Why:

This change introduces documentation for configuring terraform private registries for use by Dependabot.

/xref: https://github.com/github/dependabot-updates/issues/1480

What's being changed:

A new section was added to https://docs.github.com/en/code-security/supply-chain-security/keeping-your-dependencies-updated-automatically/configuration-options-for-dependency-updates#configuration-options-for-private-registries.

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Writer impact (This section is for GitHub staff members only):

• This pull request impacts the contribution experience
  • I have added the 'writer impact' label
  • I have added a description and/or a video demo of the changes below (e.g. a "before and after video")

[janiceilene]

@xlgmokha I see this is still in draft mode. Let me know when you're ready for a writer review 💖

[xlgmokha]

I see this is still in draft mode. Let me know when you're ready for a writer review 💖

Will do. Thanks Janice.

[xlgmokha]

I think this is ready. Back to you @janiceilene.

[asciimike]

Minor changes; otherwise, LGTM

[janiceilene]

Thanks @xlgmokha! I've triaged this for writer review ⚡

[mchammer01]

@xlgmokha - LGTM ✨

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