Docs: clarify native HTTP input scope and document Restish composition

[vmvarela]

Context

Native HTTP input was introduced in #171 and expanded to multiple URLs in #211.

sql-pipe should remain focused on:

structured input → SQLite → SQL → structured output

Native --url support is a convenience for fetching HTTP responses, not a general API or ETL client.

Scope

• Describe --url as fetching a single HTTP response.
• Keep all existing flags and behavior for compatibility.
• Document that pagination, retries, fan-out, API-specific authentication, S3 uploads, and manifests are out of scope.
• Add a recipe using Restish for paginated APIs:

restish api.example.com/items -o ndjson |
  sql-pipe -I ndjson 'SELECT id, name FROM t'

• Update the CLI help, README, and man page consistently.
• Keep Restish optional; do not add it as a dependency.
• Do not emit runtime deprecation warnings.

Acceptance Criteria

• --help describes --url as single-response convenience.
• README positions files and stdin as the primary inputs.
• README includes a Restish composition example.
• Man page documents the same product boundary.
• No HTTP implementation or behavior changes.