docs: Update preview for getting started guide for MNG and Fargate

[engedaam]

Fixes #N/A

Description

• The preview docs should reference the main branch as the is the branch where permission and API changes are pushed
• These changes impact the MNG and Fargate steps

How was this change tested?

• N/A

Does this change impact docs?

• Yes, PR includes docs updates
• Yes, issue opened: #
• No

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

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[coveralls]

Pull Request Test Coverage Report for Build 9961007379

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.01%) to 79.086%

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Files with Coverage Reduction	New Missed Lines	%
pkg/providers/amifamily/ami.go	1	91.6%

Totals
Change from base Build 9960877856:	-0.01%
Covered Lines:	5933
Relevant Lines:	7502

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💛 - Coveralls

[jmdeal]

LGTM 🚀

[jmdeal]

LGTM 🚀