docs: update and expand concept documentation pages

[Copilot]

Summary

Comprehensive overhaul of all quickli concept documentation pages across English (docs/concepts/) and German (i18n/de/.../concepts/) locales to ensure explicit behavioral specs, exhaustive capability coverage, code examples for all authoring patterns, cross-references, and Docusaurus admonitions.

Key Changes by Concept

• Overview (quickli-concepts.md): Added framework architecture map, concept hierarchy, relationship matrix, cross-references, and admonitions.
• Application (application.md): Documented all 4 command authoring/registration styles (@app.command, register_command, @app.entrypoint, load_plugin), execution models (run() vs main()), global option streams, shell completion generation, and custom error handlers.
• Command (command.md): Detailed decorator, imperative (Command), and docstring authoring styles, name normalization rules, signature binding, and Subcommand nesting hierarchies.
• Argument (argument.md): Outlined positional ordering rules, optional/default mechanics, built-in/custom validators (file_path, number_range), metavar formatting, and argument vs option decision guides.
• Option (option.md): Detailed POSIX/GNU flag forms (-vxf, --opt=val), boolean flags vs value options, repeatable list/count options (multiple=True), and global vs local scoping.
• Config (config.md): Documented TOML schema declarations (ConfigSchema, ConfigField), auto-init bootstrapping (add_auto_init_config), validation findings (error/warning), schema generation, and exception handling.
• Parsers (parsers.md): Detailed JSON/YAML/TOML serialization/deserialization helpers, argument converter integration, CLI format selection flags, and parser vs config comparison.
• Plugin (plugin.md): Documented Plugin contract (name, description, register()), load_plugin lifecycle, inspection via app.plugins, duplicate protection (PluginLoadError), and roadmap.

Code Snippet

from quickli import Application, Subcommand, positive_number

app = Application(name="demo", version="1.0.0")

# Decorator command authoring with typed arguments, options, and validators
@app.command(name="serve", description="Start service node")
def serve(
    host: str = app.argument(default="127.0.0.1", help="Bind address"),
    port: int = app.option("-p", "--port", default=8080, validator=positive_number(), help="Listen port"),
    verbose: bool = app.option("-v", "--verbose", is_flag=True, help="Verbose log output"),
):
    ...

# Imperative subcommand group registration
db_group = Subcommand(name="db", description="Database administration")
app.register_command(db_group)

Why

The existing concept documentation contained implicit assumptions, lacked coverage for alternative registration patterns and execution models, omitted cross-references between quickli building blocks, and lacked structured admonitions for key gotchas and best practices.

Validation

• python -m ruff check .
• python -m ruff format --check .
• python -m pytest
• python -m build --sdist --wheel
• Not applicable

Docusaurus production build verified cleanly via corepack pnpm --filter quickli-docs build across both en and de locales with zero broken anchor links or build warnings.

Documentation

• Documentation was updated where needed.
• No documentation update was needed.

Scope

• This pull request is focused on one feature or fix.
• Tests were added or updated for functional changes.
• Assumptions, risks, or follow-up work are described below.

Risks or follow-up

• German localized documentation (i18n/de) was synchronized concurrently; future modifications to English concept docs must maintain parity with the German localization files.

• Fixes Update concepts docs #78