docs: added missing parameter

[VishalS-Microsoft]

Purpose

This pull request adds a new parameter for configuring GPT model capacity in deployment settings and updates the relevant documentation files to reflect this change.

Documentation updates related to GPT model capacity:

• docs/CustomizingAzdParameters.md: Added the AZURE_ENV_MODEL_CAPACITY parameter to define the capacity of the GPT model, with a default value of 150.
• docs/DeploymentGuide.md: Updated the deployment guide to include the GPT Model Capacity parameter, specifying its role in setting the model's capacity with the default value of 150.

Does this introduce a breaking change?

• Yes
• No

How to Test

• Get the code

git clone [repo-address]
cd [repo-name]
git checkout [branch-name]
npm install

• Test the code

What to Check

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Other Information

[github-actions]

🎉 This PR is included in version 2.1.0 🎉

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