fix: clarify what users should do if they don't have a TENANT_NAME

[mjang]

Proposed changes

Based on user feedback. After they read current docs, they couldn't figure out how to find their TENANT_NAME.
(IMO, it's already in the docs. But we can rewrite -- and add instructions if users can't find their TENANT_NAME.)

I've set up an "include" which includes the common content in these docs. (I've linked to the build for both). Suggestion for reviewers: make sure the sections that I've linked to (now) make sense:

• Get started guide
• Secure your fleet instructions

Closes #995

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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