Section operation name: provide code example, edit text, change order…

[christiane-deneser]

… of topics

Hi there,
in my PR, I worked on Issue #241.

• I added a code example for operation names
• and edited the texts, both referring to the suggested article ( https://dev-blog.apollodata.com/the-anatomy-of-a-graphql-query-6dffa9e9e747).
• I changed the order of the topics: Section 'Operation name' now comes before section 'Variables'. I find that easier to grasp, as the examples in 'Variables' use an operation type and name, but neither had been explained before. Therefore I also adjusted the order of topics in the nav sidebar.

[stubailo]

@christiane-deneser this is wonderful, thanks!