[Nexthop] Simplify and improve PlatformManager documentation

[marif-nexthop]

Pre-submission checklist

• I've ran the linters locally and fixed lint errors related to the files I modified in this PR. You can install the linters by running pip install -r requirements-dev.txt && pre-commit install
• pre-commit run

Summary

This PR streamlines the PlatformManager documentation by reducing redundant content, improving readability, and pointing readers toward source code where appropriate. The previous PR in this series was #943.

• Consolidated Presence Detection docs - Moved the introduction into modeling.md and removed the separate page.
• Simplified Service API documentation - Reduced verbose descriptions and directed readers to the Thrift file in code instead.
• Replaced static argument lists with --help - Removed the manually maintained list of PlatformManager arguments in favor of a pointer to --help, ensuring docs don't go stale.
• Replaced inline enum lists with a link to code - Removed hardcoded enum values and linked to the source file directly.
• Improved formatting and readability - Added info and tip blocks to highlight key information and improve visual scannability. Minor wording tweaks throughout.

Test Plan

Deployed a local docusaurus instance with the new documentation and verified that changes look as expected.

[Scott8440]

Thanks for doing this!

[Scott8440]

You could link to the thrift struct for presence detection here

[marif-nexthop]

done in the latest diff.