docs: improve readme

[bra-i-am]

Description

This PR modifies the readme to have consigned the primary goal of this MFE and shows how to have it in a basic development environment.

[dcoa]

Thanks for the PR @bra-i-am

There are some points that I think should improve:

1. You need to be clear about the configuration of the platform, I mean, is a must to define VALIDATION_PANEL_MFE_URL in the settings.

2. I would rather have developing instructions like example because we use Tutor but should not be limited to that, in addition, making reference to course-authoring without an explanation will be confusing.

3. Don't forget the deploying instructions (here could be the right point to add the tutor recommendation) and use the tutor instructions to add a new MFE.

I added @MaferMazu as a reviewer understanding her wide knowledge of technical documentation.

[MaferMazu]

I will add that it would be cool to have a no-developer installation. For example: https://github.com/eduNEXT/platform-global-teacher-campus#getting-started.
Something like:

• Add X in your config.yml
• Save with tutor config save
• Create the mfe image tutor images build mfe
• Launch the platform tutor local launch

I think it's important because we probably will install this plugin more than we want to continue to develop over this, and this is not installed by default by tutor-mfe like other mfes.

Btw I only suggest; you feel free to improve my example xD

Another btw, this PR should be a "docs" instead of "refactor".