Make README more appealing

[DS-INRAE]

What this PR does / why we need it:
Add sections for readability.
Expand the text and add emojis in an attempt to make it more attractive.

Special notes for your reviewer:
In Edit File for the updated README, you can access GitHub's MD previewer

Preview at https://github.com/DS-INRA/dataverse/blob/patch-1/README.md

[DS-INRAE]

Update: I will also add a table of contents for an easier navigation and readability

[jp-tosca]

Hi @DS-INRA 👋🏼!

I was talking with @pdurbin on Zulip, you don't need HTML anchors or link to add a TOC for an MD file that lives in the repo, you can see as an example the notes for 6.3 here https://github.com/IQSS/dataverse/blob/develop/doc/release-notes/6.3-release-notes.md?plain=1

The only case where you need HTML tags is when is an MD file for the release BUT you only need the anchors:

<a id="table-of-contents"></a>
# Table of Contents
- [Release Highlights](#release-highlights)
- [Features](#features)
- [Bug Fixes](#bug-fixes)
- [API](#api)
- [Settings](#settings)
- [Complete List of Changes](#complete-list-of-changes)
- [Getting Help](#getting-help)
- [Upgrade instructions](#upgrade-instructions)

<a id="release-highlights"></a>
## Release Highlights

The links themselves don't need to be HTML, he has a point that it makes it harder to read from the console when we include HTML so it is a good practice not to add the code if it is not required/needed.