[WIP] Add user guide to customize model

[RissyRan]

Description

Add user guide to customize model, co-author with @gobbleturk

• source doc - link
• helpful for future customers to customize models for both dense and moe

Tests

Manually review in preview mode

Checklist

Before submitting this PR, please make sure (put X in square brackets):

• I have performed a self-review of my code.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have run end-to-end tests tests and provided workload links above if applicable.
• I have made or will make corresponding changes to the doc if needed.