refactor: organize images under images/

[enwaiax]

Summary

Moves every Docker image into a dedicated images/ directory so the repository has a clear boundary between website, operations, and image sources.

Layout

images/
├── maddy/
├── download-bot/
├── netease-cloud-music-tasks/
├── x-ui/
└── firefox-send/

Changes

• moves all image directories with Git rename history preserved;
• updates every submodule path in .gitmodules and verifies all five pinned commits remain unchanged;
• updates raw config/download links, project README links, website catalog metadata, contribution docs, image publishing docs, and CI path filters;
• updates the image build workflow to use images/<project> contexts;
• adds AMD64 image builds from the new locations to repository validation;
• documents the new top-level repository layout.

Compatibility note

This PR intentionally changes repository raw URLs for deployment examples. Existing users should download configs from the new images/ paths.

Verification

• git submodule status confirms all five pinned upstream commits are unchanged;
• four Dockerfiles pass Buildx checks with no warnings;
• Hadolint passes at error threshold;
• all five Compose files parse successfully;
• Astro type-check and production build pass;
• npm production audit reports 0 vulnerabilities;
• workflow files pass actionlint;
• generated site links point to images/… paths.

Not in scope

• no image content or base image upgrades;
• no Docker Hub publish;
• no submodule updates;
• no lifecycle/status changes.