docs(image loader/metadata provider): Clarify concept + how-to documentation.

[sgielen]

NOTE: This PR is currently set to merge into the beta branch. We may want to await the merge of beta into main first and then merge this PR into main afterwards?

Context

This PR improves some Custom Image Loader / Metadata Provider related documentation.

The topic of writing a Custom Image Loader based on the existing DICOM Image Loader goes back years, and I've recently taken the time to research and understand the topic a lot better. (See also #1509 for the resulting PR.) These updates to the documentation are a result of that: they represent links between documentation pages that would have made this a lot clear to begin with.

Open points

I'd like to describe the following better in documentation as well, either in this PR or in follow-up.

• Can Cornerstone display images without ANY metadata being present?
  • It looks like the IImage object has all the absolutely necessary properties, e.g. width/height, window level, pixel spacing, slice thickness. If there is no metadata at all, can Cornerstone still display the image?
  • If we need metadata, should we consider removing elements of the IImage interface, and document that these elements are instead requested via the metadata path?
• What should a metadata provider do if metadata is requested for an image that has not been loaded yet, and the metadata can only be found inside the image? (e.g. in the case of loading volumes from images)
• Should metadata still be available for an image even after decache is called on the image loader?

Changes & Results

Documentation only.

Testing

N/A

Checklist

PR

• My Pull Request title is descriptive, accurate and follows the
  semantic-release format and guidelines.

Code

• [] My code has been well-documented (function documentation, inline comments,
  etc.)

• [] I have run the yarn build:update-api to update the API documentation, and have
  committed the changes to this PR. (Read more here https://www.cornerstonejs.org/docs/contribute/update-api)

Public Documentation Updates

• The documentation page has been updated as necessary for any public API
  additions or removals.

Tested Environment

• [] "OS:
• [] "Node version:
• [] "Browser:

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[sedghi]

I love PRs that change files are all *.md ! Thanks