Update GCP credentials docs

[GGP1]

Description

Improves the GCP credentials documentation by adding a new section explaining how to create a custom role for the gcp-bucket module and a service account creation image.

Checks

Docs building

• Compiles without warnings.

Code formatting and web optimization

• Uses three spaces indentation.
• Adds or updates meta descriptions accordingly.
• Updates the redirects.js script if necessary (check this guide).

Writing style

• Uses present tense, active voice, and semi-formal registry.
• Uses short, simple sentences.
• Uses bold for user interface elements, italics for key terms or emphasis, and code font for Bash commands, file names, REST paths, and code.

[fdalmaup]

LGTM!

[javimed]

Suggested change

	To do so, follow these steps:
	To create the new role, follow these steps:

[javimed]

Suggested change

	1. Go to the **Roles** section and click on **Create Role**.
	#. Go to the **Roles** section and click on **Create Role**.

[javimed]

Suggested change

	2. Choose a name and click on **Add Permissions**.
	#. Establish a **Title** and click on **Add Permissions**.

[javimed]

Suggested change

	3. On the search bar, type **Storage Legacy Bucket Writer** and select the following permissions:
	#. On the search bar, filter available permissions by typing **Storage Legacy Bucket Writer**. Select the following ones:

[javimed]

Suggested change

	The ``gcp-bucket`` module requires permissions to access storage buckets and objects. These permissions were originally assigned using the **Storage Legacy Bucket Writer** role, but it is no longer available for selection, so it will be necessary to create a new one.
	The ``gcp-bucket`` module requires permissions to access storage buckets and objects. To create a role with the required permissions, follow these steps:

[javimed]

Suggested change

To create the new role, follow these steps: