[Docs] Working with a Federated Graph

[JakeDawkins]

This is the docs overview for the rover subgraph command subset

should include:

• note that this is not the right command to use when working with non-federated graphs (and link to that).
• link to apollo federation docs
• 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 subgraph push my-graph --service-name accounts --schema ./schema.graphql
rover subgraph introspect http://localhost:4000 | rover subgraph push my-graph@dev -s - --service-name accounts --routing-url http://hi.dev
rover subgraph check my-graph -s ./schema.graphql --service-name accounts
rover subgraph introspect http://localhost:4000 | rover subgraph push my-graph@dev -s - --service-name accounts --routing-url http://hi.dev
rover subgraph fetch my-graph@dev0 --service-name accounts > schema.graphql