Update and improve `IAM Roles` section

[Selutario]

Description

The IAM Roles section of the Configuring AWS credentials page is a bit confusing for some reasons. Some are:

1. Previously created policy

This is mentioned in the step number 4:

Select the previously created policy:

However, none of the previous steps mention anything about creating policies or what they should contain. The step refers to the policies that are created under the heading Policy configuration of each of the supported services (for example, for Cloudtrail), but without further description and taking into account that the credential configuration is part of the Prerequisites (it will be read before the Policy configuration), it is difficult for a user to know what to do here.

We should clarify, either in this step or in an initial step within IAM Roles, that it is mandatory to create a specific policy depending on the service that will later be configured, and also where is located the information to create said policy.

2. sts:AssumeRole

The step number 7 says:

7. Add permissions so the new role can do sts:AssumeRole action:

Some parts of the screenshot are pixelated so as not to reveal information, however, this makes it difficult for the user to understand what they should write there, in addition to the fact that it is not explained in the step description.

3. Outdated screenshots

Most of the screenshots in this section are outdated, we should update them.