docs: Add missing index for reference API

[ribdp]

Pull Request Description

1. Add reference api index.rst for driver documentation

PR Type

• Bug fix (change that fixes an issue)
• New feature (change that adds new functionality)
• Breaking change (has dependencies in other repos or will cause CI to fail)

PR Checklist

• I have performed a self-review of the changes
• I have commented my code, at least hard-to-understand parts
• I have build all projects affected by the changes in this PR
• I have tested in hardware affected projects, at the relevant boards
• I have signed off all commits from this PR
• I have updated the documentation by running /docs/source/reference_api/gen_sysobj_doc.m script locally, if applies.