Add a guide for migrating a fieldmanager field to using gutenberg

[fabiankaegy]

We often inherit legacy codebases that use field manager fields. For this to work we need to register the meta field to be exposed to the rest API and then we can use it as intended.

@ajmaurya99 is just not working on this on a project so if you have the time I'd love for you two create a quick guide for it :)