feat: Rework Manifest installation cases

[ADubhlaoich]

Proposed changes

This commit reworks the manifest installation document to instead become
a folder, with each individual page within covering exactly one use
case. The new structure guides a user completely from end to end on the
process necessary for installing NGINX Gateway Fabric NGINX Open Source
or NGINX Plus without using multiple documents.

It also includes general improvements to specification discoverability,
allowing readers to more easily find what parts of the Gateway API
specification are supported and what to expect from future releases.

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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---

[ADubhlaoich]

Pushed my latest changes.

Running into issues testing the NGINX Plus deployment: there's no problems in the actual installation process, but the pod never becomes ready due to a crash loop.

The nature of the deployment makes me assume it could be related to the license for the pod.

[ADubhlaoich]

The Plus deployment case now works as expected: I'd been trying to create consistency with Gateway Fabric and Ingress Controller processes.

The latter installs everything (including secrets) to the default namespace, whereas NGINX Gateway Fabric confines it all to its own namespace.

[ADubhlaoich]

After conferring with the rest of the documentation team, I will be preparing whole documents for the experimental use case, which will be applied to other deployments as part of a design pattern.

I have yet to test the nuances of the cert-manager installation, and will do so after the content work.

[ADubhlaoich]

Discovered a missing parameter from the old Manifest documentation when installing the API resources: adding it to the new use case resolves as expected.

Finished testing the open source experimental deployment: tearing down my test deployment and trying the plus experimental deployment use case now.

[ADubhlaoich]

Found an incorrect URL for the NGINX Plus experimental deployment: works as expected!

All that's left is testing the Let's Encrypt certificate documentation to see if it works with a "default" contemporary cert-manager installation, though one of the requirements for testing is... challenging.

[sjberman]

Suggested change

	```text
	NAME TYPE DATA AGE
	nplus-license Opaque 1 31s
	nginx-plus-registry-secret kubernetes.io/dockerconfigjson 1 22s
	```
	```text
	NAME TYPE DATA AGE
	nplus-license Opaque 1 31s
	nginx-plus-registry-secret kubernetes.io/dockerconfigjson 1 22s

[sjberman]

Suggested change

	```text
	NAME TYPE DATA AGE
	nplus-license Opaque 1 31s
	nginx-plus-registry-secret kubernetes.io/dockerconfigjson 1 22s
	```
	```text
	NAME TYPE DATA AGE
	nplus-license Opaque 1 31s
	nginx-plus-registry-secret kubernetes.io/dockerconfigjson 1 22s