docs: major documentation overhaul

[Copilot]

The documentation was a mixed bag of incomplete class references and sparse descriptive content with many empty sections. This rewrites it from scratch with clear separation between introductory material, practical guides, and API reference.

New Structure

• Introduction — Purpose, architecture, design principles, ecosystem overview
• Getting Started — Installation, minimal conversion example, pointers to guides
• User Guides:
  • Converting Sigma rules into queries (collections, pipelines, output formats, correlation rules)
  • Building a backend (TextQueryBackend class variables, output formats, finalize_query, testing)
  • Writing processing pipelines in Python and YAML (priorities, all transformations/conditions, post-processing, finalizers, custom transformations)
  • Validating rules (YAML config, built-in validators, writing custom validators)
  • Plugin system (namespace packages, backend/pipeline/validator plugin structure)
• API Reference — Full autodoc coverage of all 12 module groups
• TODO (docs/todo.rst) — Remaining work tracked (correlation guide, filters guide, migration guide, etc.)

Config & Code Changes

• docs/conf.py: Added napoleon, viewcode, intersphinx extensions
• sigma/processing/transformations/external.py: Reformatted 3 docstrings (FilePlaceholder, HTTPPlaceholder, CommandPlaceholder) from malformed bullet lists to NumPy-style params for clean Sphinx rendering

Example

from sigma.rule import SigmaRule
from sigma.collection import SigmaCollection
from sigma.backends.test import TextQueryTestBackend

rule = SigmaRule.from_yaml("""
    title: Test Rule
    status: test
    logsource:
        category: process_creation
        product: windows
    detection:
        selection:
            CommandLine|contains: mimikatz
        condition: selection
""")

backend = TextQueryTestBackend()
result = backend.convert(SigmaCollection(rules=[rule]))

Builds with 0 Sphinx errors. All 1511 tests pass. mypy/black clean.