docs: add Introduction section to Getting Started page

[bedus-creation]

Summary

Adds a new ## Introduction section at the top of the Getting Started page, before ## Prerequisites.

The section frames why the framework exists:

• FastAPI stops at the request/response boundary and leaves app-level concerns (config, logging, DB, DI, console) to you.
• Teams end up copy-pasting the same bootstrap across microservices, and those copies drift.

It then shows how an application is composed by registering providers, with a short Application(providers=[...]) example.

The previous one-line opening paragraph is folded in and refined into the new section.

Scope

• Only docs/getting-started.md is changed.

[bedus-creation]

Code review

No blocking issues found. Checked for factual accuracy and consistency. Verdict: approve.

Verified:

• The Application(base_path=..., providers=[FastAPIProvider]) example matches the real framework API — Application.__init__ accepts both base_path= and providers=, and both from fastapi_startkit import Application and from fastapi_startkit.fastapi import FastAPIProvider are valid exports.
• The ## Introduction section is placed as the first H2 before ## Prerequisites, matching task #642's acceptance criteria.
• Prose accurately frames the framework's purpose (app-level concerns beyond FastAPI's request/response boundary, provider-driven composition).

Non-blocking notes (optional, not required for merge):

• The intro snippet previews the same Application + FastAPIProvider pattern shown later in "Way 1"/"Way 2". This is intentional introductory scaffolding, but if you want to trim redundancy you could shorten the intro block to prose plus a forward reference.
• Heads-up: open PR docs(getting-started): add "Getting Started Using AI" section #55 edits the same opening region and will likely produce a merge conflict — whichever merges second will need a rebase.

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