Add documentation to the `docs.fuel.network` about how `fuel-core` works

[xgreenx]

• Describe how mempool works
• Executor
  • Upgradable Executor vs Native (maybe explain how-to-upgrade here)
  • Production vs. DryRun vs Validation (maybe explain production nodes vs sentry nodes vs any other nodes)
• API ...TODO

[MitchTurner]

This needs more clarification :)

Who is the audience of these docs? It sounds like it's a semi-technical audience. They probably don't need to know in-depth how everything works internally, but rather the sorts of things an external person might want to understand, e.g. API, Networking, Upgrading, etc.