chore: add conf.d example split-config files

Three example files (alpine, github, pypi) demonstrating per-remote
YAML files for the conf.d directory mode.

conf.d/alpine.yaml

@@ -0,0 +1,11 @@
+remotes:
+  alpine:
+    base_url: "https://dl-cdn.alpinelinux.org"
+    type: "remote"
+    package: "alpine"
+    description: "Alpine Linux APK package repository"
+    immutable_patterns:
+      - ".*/x86_64/.*\\.apk$"
+    cache:
+      immutable_ttl: 0
+      mutable_ttl: 7200

conf.d/github.yaml

@@ -0,0 +1,12 @@
+remotes:
+  github:
+    base_url: "https://github.com"
+    type: "remote"
+    package: "generic"
+    description: "GitHub releases and files"
+    immutable_patterns:
+      - "gruntwork-io/terragrunt/.*terragrunt_linux_amd64.*"
+      - "prometheus/node_exporter/.*/node_exporter-.*\\.linux-amd64\\.tar\\.gz$"
+    cache:
+      immutable_ttl: 0
+      mutable_ttl: 0

conf.d/pypi.yaml

@@ -0,0 +1,17 @@
+remotes:
+  pypi:
+    base_url: "https://files.pythonhosted.org"
+    type: "remote"
+    package: "pypi"
+    description: "Python Package Index"
+    check_mutable_updates: true
+    quarantine_new: true
+    quarantine_days: 3
+    immutable_patterns:
+      - "packages/.*\\.whl$"
+      - "packages/.*\\.whl\\.metadata$"
+      - "packages/.*\\.tar\\.gz$"
+      - "packages/.*\\.zip$"
+    cache:
+      immutable_ttl: 0
+      mutable_ttl: 600