Add a `How it works` section

[ra0x3]

• This section would address the following documentation issues:

  • [ ] https://github.com/FuelLabs/fuel-indexer/issues/1490
  • [ ] https://github.com/FuelLabs/fuel-indexer/issues/1475
  • [ ] https://github.com/FuelLabs/fuel-indexer/issues/1476
  • [ ] https://github.com/FuelLabs/fuel-indexer/issues/1491
  • [ ] https://github.com/FuelLabs/fuel-indexer/issues/1500
    • You can't redefine scalar types...

• It would be placed below "About" and above "Getting started"

• This would be a pretty long section with link navigation at the top, and each section broken down with headers and examples

• Sections should probably be:

  • "At a high level"
    • Brief summary of how every thing works (compilation, deployment, registration, execution, DB interaction)
  • Compilation
    • Describe how indexers are compiled
  • Deployment
    • Describe how indexers are deployed
      • With all of the various options
  • Registration
    • Describe how deployed indexer assets are registered to the service
  • Execution
    • Describe the process of an indexer getting new data and calling handlers on that data
      • Maybe talk about some of the useful/different start args passed to fuel-indexer
  • DB interaction
    • Describe how indexers save data to the database

• Every section should include minimal functional examples or diagrams of what the section is describing