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/generic/remote.md

@@ -0,0 +1,26 @@
+# `generic` remote
+
+| Type | Mutable (auto-detected) | Immutable (auto-detected) |
+|---|---|---|
+| `generic` | nothing | everything |
+
+## Terraform
+
+```hcl
+resource "artifactapi_remote_generic" "github" {
+  name     = "github"
+  base_url = "https://github.com"
+
+  immutable_ttl = 0
+  mutable_ttl   = 7200
+
+  patterns = [
+    "ducaale/xh/.*/xh-.*-x86_64-unknown-linux-musl.tar.gz$",
+    "mikefarah/yq/.*/yq_linux_amd64$",
+  ]
+
+  mutable_patterns = [
+    ".*/archive/refs/heads/.*\\.tar\\.gz$",
+  ]
+}
+```