split per-type registration points into per-type files

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

docs/docker/remote.md

@@ -0,0 +1,24 @@
+# `docker` remote
+
+| 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",
+  ]
+}
+```