[Docs] Working with Graphs

[JakeDawkins]

This is the docs overview for the rover graph command subset

should include:

• note that this is not the right command to use when working with federated graphs (and link to that)
• rover graph --help instruction to see full api reference
• List of subcommands and their purpose (like a sentence or two each)
• "Recipes" or example usages, for ex:

rover graph push my-graph --schema ./schema.graphql
rover graph introspect http://localhost:4000 | rover graph push my-graph@dev -s -
rover graph check my-graph -s ./schema.graphql
rover graph introspect http://localhost:4000 | rover graph push my-graph@dev -s -
rover graph fetch my-graph@dev0 > schema.graphql