RHOAIENG-3943 Update Model Registry documentation

[tarilabs]

Description

I followed same style per https://issues.redhat.com/browse/RHOAIENG-3946

I've used as sources:

• "Model Registry Architecture"
• "Model Registry Proposal"
• https://issues.redhat.com/browse/RHOAIENG-1071 
• https://github.com/opendatahub-io/model-registry-bf4-kf/issues/249 
• Implement model registry inference service reconciliation odh-model-controller#135

Since "Model Registry Tenancy Proposal" is still under finalization I didn't include it yet in the current PR, we can expand on it later once the ADR is approved.

How Has This Been Tested?

Merge criteria:

• The commits are squashed in a cohesive manner and have meaningful messages.
• Testing instructions have been added in the PR body (for PRs involving changes that are not immediately obvious).
• The developer has manually tested the changes and verified that the changes work

[etirelli]

made some minor editorial comments, but I suggest rewording the whole document to reflect what is the model server component (present tense), instead of what is intended for the future.

[tarilabs]

made some minor editorial comments, but I suggest rewording the whole document to reflect what is the model server component (present tense), instead of what is intended for the future.

thank you for the feedback @etirelli , I believe I rewritten all future tense sentences I could find to present tense with 4dd3c07