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 
• https://github.com/opendatahub-io/odh-model-controller/pull/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

[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 https://github.com/opendatahub-io/architecture-decision-records/pull/34/commits/4dd3c077b0813126372aa2044da47b7931cd6f31