[DOC] doc update for gluon artifact support

[liusy182]

Description

Follow up on comments from #1264 , this PR updates documentation for the newly added gluon artifacts.

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 #1271 (7ee8137) into master (f9ae334) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1271   +/-   ##
=======================================
  Coverage   66.30%   66.30%           
=======================================
  Files         143      143           
  Lines        9314     9314           
=======================================
  Hits         6176     6176           
  Misses       3138     3138           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f9ae334...7ee8137. Read the comment docs.

[parano]

Thank you @liusy182!