feat: Update NGINX Gateway Fabric information architecture

[ADubhlaoich]

Proposed changes

This pull request partially restructures the NGINX Gateway Fabric documentation to flatten the folder structure, reducing the amount of steps a reader must navigate to get to useful instructions.

The majority of the effort is concentrated on the Install section, simplifying the URL structure and creating logical links in the sequence of pages presented and the linear connection of pre-requisites and next steps when a user is onboarding a cluster.

Additional work will follow this effort, once identifying more common user journeys and how individual documents can be grouped together logically.

Checklist

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• I have read the contributing guidelines
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• If the change involves potentially sensitive changes¹, I have assessed the possible impact
• If applicable, I have added tests that prove my fix is effective or that my feature works
• I have ensured that existing tests pass after adding my changes
• If applicable, I have updated README.md and CHANGELOG.md

Footnotes

1. Potentially sensitive changes include anything involving code, personally identify information (PII), live URLs or significant amounts of new or revised documentation. Please refer to our style guide for guidance about placeholder content. ↩

[github-actions]

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[sjberman]

FYI the docs preview link doesn't work once I click into specific folders. The URL changes and doesn't reference properly.

[ADubhlaoich]

@sjberman Investigating this issue now...

[sjberman]

I like this new layout.

We will also have to update links in our NGF repo that currently point to these docs.

[salonichf5]

FYI the docs preview link doesn't work once I click into specific folders. The URL changes and doesn't reference properly.

I did make watch locally and all the pages reflected correctly and links were working

[bjee19]

lgtm after some small changes

[ADubhlaoich]

Pushed a change to address the feedback, though there's still one ongoing conversation.

I've also updated all of the files to add a prefix for our custom frontmatter data and removed horizontal lines in advance of changes in our documentation website's design and cataloguing systems.

There is follow up work after this PR to update link references in the NGF repository per some work Saylor is doing for the release.