docs: Add redirect link for Block Devices

[lanandra]

Description

Add redirect link for Block Devices configuration block details

Motivation and Context

Previously the description stated as below

Customize details about the root block device of the instance. See Block Devices below for details

I checked in the README there is no information details regarding Block Devices thus this will make word below misleading. As a solution, propose to change that with redirect link to AWS provider documentation section Block Devices in resource aws_instance

Breaking Changes

No breaking changes

How Has This Been Tested?

• [ ] I have updated at least one of the examples/* to demonstrate and validate my change(s)
• [ ] I have tested and validated these changes using one or more of the provided examples/* projects

• [ ] I have executed pre-commit run -a on my pull request