Add Table Of Content for Migration Guide

[matks]

Table of Content was generated using https://tableofcontents.herokuapp.com/

It is useful when you are in the middle of a Symfony migration and you want to go back to a part of the documentation somewhere in the Markdown document. It avoids the need to scroll up and down, looking for the part you want.

[mickaelandrieu]

Good idea @matks, but we need to update it everytime we add a new section to this file.