Add nested side navigation to documentation for client methods

[tim-finnigan]

Describe the issue

Recent improvements to the boto3 docs have received lots of positive feedback (credit to @jonathan343 for leading the effort there!)

One more enhancement to consider is adding nested side navigation for client methods. This would make it easier to jump between different commands when you the documentation up for a specific service.

I wanted to create a tracking issue for this because I've now seen it suggested a couple times in other places:

• https://www.reddit.com/r/aws/comments/urvf4g/aws_i_love_your_services_and_apis_but_your/
• https://www.reddit.com/r/aws/comments/135ltbq/comment/jikmt8n/?utm_source=share&utm_medium=web3x&utm_name=web3xcss&utm_term=1&utm_content=share_button

Links

https://boto3.amazonaws.com/v1/documentation/api/latest/reference/services/index.html

[tim-finnigan]

Somewhat related feature request: #3672 (although that is asking specifically for nested navigation for the subheadings like Request Syntax, Response Syntax, etc. on an individual page).