feat: split struct org data from SeoMixin

[Chadys]

Fix #60 I put every organization structured data inside of SeoSettings and left the utility classes to access that data inside of SeoMixin.

I split the template for structured data in two so that structured org data is not fetched for every pages while still keeping articles struct data by default. This can be discussed.

I updated the tests and the documentation as necessary, but I am still not satisfied with this solution; I think moving from page to settings for struct data makes the upgrade a much too heavy process for current users.

I will make a second PR to test the solution of the SeoOrgMixin that we discussed, I think it will be much more upgrade friendly, tell me what you think :)