docs: Update Terragrunt custom workflows docs

[Alex-Mussell]

• Docs to implement target plan, import, and state removal workaround for Terragrunt

what

• Documentation updated to include a full Terragrunt custom workflow that has the capabilities to use the commands not supported natively for Terraform wrappers.

why

• By default, Atlantis doesn't support targetted plans/applies, imports, and state removes for Terragrunt out of the box. This update provides a workaround that works, and other people might find useful

tests

• N/A

references

• No supporting issue number. However this was discussed in the community Slack.

[jamengual]

@Alex-Mussell Thanks for the contribution