Split per-type registration points into per-type files (#150)

Parallel package-type PRs conflict because every type edits the same shared lines (README tables, provider.go, server.go imports, the package type list, UI maps, the e2e caching table). A new type now only adds files.

- move per-type README sections into docs/<type>/{remote,local,virtual}.md
- split provider.go capability and metadata types into cap_*.go and metadata_*.go
- import providers via internal/provider/all, one file per type
- declare each package type in package_type_<type>.go, registered via init()
- split the e2e caching cases and UI colours/snippets into per-type files

Reviewed-on: #150
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

docs/docker/remote.md

@@ -0,0 +1,26 @@
+# `docker` remote
+
+Shared classification, TTL and eviction rules: [caching](../common/caching.md).
+
+| Type | Mutable (auto-detected) | Immutable (auto-detected) |
+|---|---|---|
+| `docker` | tag manifests, `/tags/list` | blobs, digest manifests |
+
+## Terraform
+
+```hcl
+resource "artifactapi_remote_docker" "dockerhub" {
+  name     = "dockerhub"
+  base_url = "https://registry-1.docker.io"
+
+  immutable_ttl    = 0
+  mutable_ttl      = 300
+  ban_tags_enabled = true
+  ban_tags         = ["latest"]
+
+  patterns = [
+    "^library/postgres",
+    "^library/redis",
+  ]
+}
+```