docs: clearly explain dev, build, local test, local test with Firebase Emulator and production deployment flows

[jthegedus]

Is your feature request related to a problem? Please describe.

There are 5 stages during the dev > build > test > deploy cycle for SvelteKit apps. These can be confusing to users; what capabilities are utilised or available in each step?

These are the steps:

1. dev: svelte-kit dev
2. build prod (used for all subsequent steps): svelte-kit build
3. test prod build locally without adapter environment: svelte-kit preview
4. test prod build locally with adapter environment: see adapter docs
5. deploy prod build to adapter environment: see adapter docs

Describe the solution you'd like

Summary each step and their purpose in a succinct way.

Describe in detail how to use each step where the adapter applies.

Describe alternatives you've considered

NA

Additional context

The official docs are going through improvements in this area, but documentation is required here to explain the functions of the Firebase adapter int his process.