📖 add notes on building oci artifacts as well as using plain http

[amsuggs37]

What this PR does / why we need it:
This PR adds a few notes to the air-gapped-environment documentation that add clarity to properly configuring the oci artifacts to be fetched.
I was recently setting up an air-gapped installation of the operator and installing providers to my cluster following these docs and found that it was ambiguous on how to configure the artifacts. I was under the impression that the artifacts need only to be in an oci compliant registry rather than the oci layout. As such, I was building simple docker images that were not in oci layout and the operator could not find the metadata/components files for any given provider despite successfully fetching the artifact from the oci registry.

The added notes include a link to the local plugin docs describing how to properly build and publish provider artifacts such that the operator can successfully fetch the artifact and then also extract the metadata/components files. I also add a note on how to fetch artifacts using plain http. As I was in development, my oci registry was a local zarf registry and I was accessing it over http. Using a fetchConfig.oci without the http:// prefix resulted in the operator erroring with a message stating that the server returned an http status to an https client. I had to dig through the controller source code to find out how to properly instruct the operator to pull the artifacts via http.

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[k8s-ci-robot]

Hi @amsuggs37. Thanks for your PR.

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[furkatgofurov7]

/ok-to-test
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: furkatgofurov7

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[alexander-demicev]

/lgtm

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: 8c370b3af2b9b3509d13aeaa28605e3626ad92db