[doc] fix header level warnings from CI soundness/doc scripts

[sebsto]

note: The majority of content should be under level-3 headers under the "Overview" section
  --> Deployment.md:22:1-22:17
20 |   * [Third-party tools](#third-party-tools)
21 |
22 + ## Prerequisites
   | ╰─suggestion: Change the title to "Overview"
23 |
24 | 1. Your AWS Account

and many others.

Action: I lowered all titles one level down.

[Copilot]

Pull Request Overview

This PR fixes documentation header level warnings from CI soundness/doc scripts by lowering all header titles by one level to comply with documentation structure requirements. The changes ensure proper hierarchical organization under the expected "Overview" section structure.

• Lowered all documentation headers by one level (## to ###, ### to ####, etc.)
• Added missing "Overview" section in Deployment.md
• Updated documentation parameter description in LambdaResponseStreamWriter

Reviewed Changes

Copilot reviewed 5 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
scripts/check-doc.sh	New documentation checking script that validates documentation targets and enforces header structure warnings
Sources/AWSLambdaRuntime/LambdaHandlers.swift	Added missing parameter documentation for hasCustomHeaders parameter
Sources/AWSLambdaRuntime/Docs.docc/quick-setup.md	Lowered "High-level instructions" from ## to ###
Sources/AWSLambdaRuntime/Docs.docc/Proposals/0001-v2-api.md	Systematically lowered all headers by one level throughout the proposal document
Sources/AWSLambdaRuntime/Docs.docc/Deployment.md	Added "Overview" section and lowered all headers by one level

Comments suppressed due to low confidence (1)

scripts/check-doc.sh:46

• The swift-docc-plugin version "1.0.0" may not exist. The swift-docc-plugin was released with different version numbering. Consider using a valid version like "1.4.3" or checking the actual available versions.

    .package(url: "https://github.com/swiftlang/swift-docc-plugin", from: "1.0.0")