docs: Revert node groups deletion

[Zvikan]

What does this PR do?

• Revert deletion of node groups docs

🛑 Please open an issue first to discuss any significant work and flesh out details/direction - we would hate for your time to be wasted.
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Motivation

Post https://github.com/aws-ia/terraform-aws-eks-blueprints/pull/1104/files which lead to #1142, later on we reverted the node groups examples (#1121) but forgot to revert the docs for the node groups.

More

• Yes, I have tested the PR using my local account setup (Provide any test evidence report under Additional Notes)
• Yes, I have added a new example under examples to support my PR
• Yes, I have created another PR for add-ons under add-ons repo (if applicable)
• Yes, I have updated the docs for this feature
• Yes, I ran pre-commit run -a with this PR

Note: Not all the PRs require a new example and/or doc page. In general:

• Use an existing example when possible to demonstrate a new addons usage
• A new docs page under docs/add-ons/* is required for new a new addon

For Moderators

• E2E Test successfully complete before merge?

Additional Notes

[edmiranda]

@Zvikan thanks for opening this PR

[vara-bonthu]

Thanks for adding the docs 👍🏼