docs: rewrite Configuration.md (remove plugin tree, add Database/crawl limits, redirect to abx-plugins)

This rewrite (now reapplied on top of the wiki subtree) covers the full
session's work on Configuration.md:

- Add crawl/snapshot limits (CRAWL_MAX_URLS/SIZE/TIMEOUT,
  CRAWL_MAX_CONCURRENT_SNAPSHOTS, SNAPSHOT_MAX_SIZE), DELETE_AFTER,
  PERMISSIONS, PLUGINS/ENABLED_PLUGINS/ACTIVE_PERSONA.
- Add new Database Settings section (SQLITE_* tuning + DATABASE_NAME).
- Add SERVER_SECURITY_MODE deep-dive (4 modes, host-layout table).
- Add Storage path overrides (DATA_DIR, ARCHIVE_DIR, USERS_DIR,
  PERSONAS_DIR, CRAWL_DIR, SNAP_DIR, ALLOW_NO_UNIX_SOCKETS).
- Remove ALLOWED_HOSTS + CSRF_TRUSTED_ORIGINS as user-settable; both
  auto-derived from BASE_URL + SERVER_SECURITY_MODE. Backward-compat
  anchors preserved on BASE_URL with the 0.7.3 -> 0.9 legacy upgrade note.
- Remove the entire Plugin Settings tree (~200 options, 41 subsections);
  replace with prominent redirect to https://archivebox.github.io/abx-plugins/
  and a "shared core options that plugins fall back to" table.
- Add 231 backward-compat <a id="..."></a> anchors so old URLs to plugin
  sections / removed options / multi-option headers all still resolve
  (e.g. #wget_args -> Plugin Configuration section, #public_snapshots ->
  PERMISSIONS, #ssl_enabled -> Plugin Configuration, #admin_username ->
  ADMIN_USERNAME/PASSWORD heading, #dir_output_permissions ->
  OUTPUT_PERMISSIONS, #url_blacklist -> URL_DENYLIST).
- Fix wrong default: PUBLIC_ADD_VIEW is False, not True.
- Drop the 7 TRAFILATURA_OUTPUT_* per-format flags (replaced by single
  TRAFILATURA_OUTPUT_FORMATS in plugin); SSL_ENABLED/SSL_TIMEOUT (wrong
  plugin namespace) — anchors redirected to Plugin Configuration.
- Reframe COOKIES_FILE as low-level escape hatch; personas are the
  preferred auth path.
- Link every named plugin to its specific anchor on the abx-plugins page
  (e.g. WGET_TIMEOUT -> #wget, SONIC_HOST -> #search_backend_sonic).
- Strip implementation-detail mentions (Pydantic, etc.).
- Slim Shell Options to only user-settable (DEBUG, USE_COLOR,
  SHOW_PROGRESS); drop IS_TTY/IN_DOCKER/IN_QEMU.
- Restructure: General -> Server (+LDAP) -> Storage -> Database (new) ->
  Search -> Shell -> Plugin Configuration.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>