[DOCS] Publish available APIs

[wiwaszko-intel]

Description

Make use of the existing and available API yamls to include in the published documentation.

• Model Download: Add api-reference.md with Swagger plugin (instead of a direct download of openapi.yaml), move openapi.yaml to _assets (to conform with the OEP docs structure template), update refs in Dockerfile and others
• Multi-level Video Understanding: add Swagger plugin to api-reference.md
• Chat QnA Core: nesting issue fix for chatqna-api.yml

How Has This Been Tested?

Built locally and viewed with Live Server to verify APIs render on the site.

Checklist:

• I agree to use the APACHE-2.0 license for my code changes.
• I have not introduced any 3rd party components incompatible with APACHE-2.0.
• I have not included any company confidential information, trade secret, password or security token.
• I have performed a self-review of my code.