docs: add AWSIM integration page

[ismetatabay]

Description

This pull request adds an AWSIM integration page that links to relevant AWSIM documentation sections, including instructions on adding new vehicles, creating environments, and setting up a Unity project.

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[github-actions]

Documentation URL: https://autowarefoundation.github.io/autoware-documentation/pr-487/
Modified URLs:

• https://autowarefoundation.github.io/autoware-documentation/pr-487/how-to-guides/integrating-autoware/awsim-integration/

[mitsudome-r]

We could potentially combine this with AWSIM documentation in the future as we do the refactoring.