[Docs]: Cloud9 is no longer available in newly created accounts

### Describe the documentation issue

Given this blog [announcement](https://aws.amazon.com/blogs/devops/how-to-migrate-from-aws-cloud9-to-aws-ide-toolkits-or-aws-cloudshell/) AWS no longer offer the Cloud9 service in new accounts. When following the installation guide using the Cloud9 Instructions users will run into the following error:

"This account does not have access to the Cloud9 service
For capabilities similar to AWS Cloud9, explore AWS Toolkits in your own IDE and AWS CloudShell in the AWS Management Cons"

The docs should make this clear to new users, and provide alternate instructions now that Cloud9 is deprecated.

### Links

https://github.com/awslabs/aws-deployment-framework/blob/master/docs/installation-guide.md#22-cloud9-instructions

### Acknowledgements

- [ ] I may be able to submit a pull-request to fix this issue.

Read docs/installation-guide.md, especially the Cloud9 Instructions section, and review the linked AWS announcement about migrating from Cloud9. Done means new users are warned that Cloud9 is unavailable in newly created accounts and are given documented alternative instructions.

Written by the indexing model from the issue text.