[Doc]Add model artifact metadata information to the core concept page

[yubozhao]

Description

Motivation and Context

How Has This Been Tested?

Types of changes

• Breaking change (fix or feature that would cause existing functionality to change)
• New feature and improvements (non-breaking change which adds/improves functionality)
• Bug fix (non-breaking change which fixes an issue)
• Code Refactoring (internal change which is not user facing)
• Documentation
• Test, CI, or build

Component(s) if applicable

• BentoService (service definition, dependency management, API input/output adapters)
• Model Artifact (model serialization, multi-framework support)
• Model Server (mico-batching, dockerisation, logging, OpenAPI, instruments)
• YataiService gRPC server (model registry, cloud deployment automation)
• YataiService web server (nodejs HTTP server and web UI)
• Internal (BentoML's own configuration, logging, utility, exception handling)
• BentoML CLI

Checklist:

• My code follows the bentoml code style, both ./dev/format.sh and
  ./dev/lint.sh script have passed
  (instructions).
• My change reduces project test coverage and requires unit tests to be added
• I have added unit tests covering my code change
• My change requires a change to the documentation
• I have updated the documentation accordingly

[codecov]

Codecov Report

Merging #1259 (f90edb2) into master (8250f87) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1259   +/-   ##
=======================================
  Coverage   66.09%   66.09%           
=======================================
  Files         141      141           
  Lines        9173     9173           
=======================================
  Hits         6063     6063           
  Misses       3110     3110           

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[jackyzha0]

oops, should've included this in my PR 😅 thanks!

[jackyzha0]

might be more clear to put this above the code block

[yubozhao]

Yeah, might be. I am trying to catch readers' attention on this. Maybe have the message in the code is more natural.