NGINXaaS: add docs for Secret Manager integration

[valyria257]

Proposed changes

NGINXaaS can now fetch secrets from Secret Manger. This adds
instructions on how to integrate with Secret Manager and use
secrets as certificates for NGINX configurations.

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This moves the "Identity and access management" page to
be under the "Getting started" section because it no
longer applied to just logging and monitoring. Luckily,
the page's URL is already under getting-started, so that
did not need to change.

This also updated the page's contents to add instructions
to allow fetching secrets from Secret Manager.

Checklist

Before sharing this pull request, I completed the following checklist:

• I read the Contributing guidelines
• My branch adheres to the Git conventions
• My content changes adhere to the F5 NGINX Documentation style guide
• If my changes involve potentially sensitive information¹, I have assessed the possible impact
• I have waited to ensure my changes pass tests, and addressed any discovered issues

Footnotes

1. Potentially sensitive information includes personally identify information (PII), authentication credentials, and live URLs. Refer to the style guide for guidance about placeholder content. ↩

[github-actions]

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[JTorreG]

lgtm, minor nits

[amudukutore]

Had a couple of minor comments (nothing of real importance - feel free to ignore). Thanks for making these changes.