message: Fix: incorrect nydusd config
  Keep `backend` and `cache` fields in `device`, move other fields to outside;
  Fix incorrect JSON syntax rendering for Github markdown;
diff: @@ -69,15 +69,14 @@ mount -t virtiofs nydus /mnt
  #### Common Fields In Config
  -``` JSON
  +```
  {
  "device": {
  "backend": {
  // localfs | oss | registry
  - "type": "...",
  + "type": "localfs",
  "config": {
  - ...
  - // Access remote storage backend via P2P proxy, for example Dragonfly client address
  + // Access remote storage backend via P2P proxy, e.g. Dragonfly client
  "proxy": "http://p2p-proxy:65001",
  // Fallback to remote storage backend if P2P proxy ping failed
  "proxy_fallback": true,
  @@ -90,7 +89,8 @@ mount -t virtiofs nydus /mnt
  // Drop the read request once http connection timeout, in seconds
  "connect_timeout": 5,
  // Retry count when read request failed
  - "retry_limit": 0
  + "retry_limit": 0,
  + ...
  }
  },
  "cache": {
  @@ -103,6 +103,7 @@ mount -t virtiofs nydus /mnt
  // Directory of cache files, only for blobcache
  "work_dir": "/cache"
  }
  + }
  },
  // direct | cached
  "mode": "direct",
  @@ -117,11 +118,9 @@ mount -t virtiofs nydus /mnt
  "enable": false,
  // Prefetch thread count
  "threads_count": 10,
  - // Maximal read size per prefetch request, for example 128kb
  + // Maximal read size per prefetch request, e.g. 128kb
  "merging_size": 131072
  }
  - },
  - ...
  }
  ```
  @@ -129,7 +128,7 @@ mount -t virtiofs nydus /mnt
  ##### Localfs Backend
  -``` JSON
  +```
  {
  "device": {
  "backend": {
  @@ -151,12 +150,13 @@ mount -t virtiofs nydus /mnt
  ##### OSS backend with blobcache
  -``` JSON
  +```
  {
  "device": {
  "backend": {
  "type": "oss",
  "config": {
  + ...
  "endpoint": "region.aliyuncs.com",
  "access_key_id": "",
  "access_key_secret": "",
  @@ -171,18 +171,18 @@ mount -t virtiofs nydus /mnt
  ##### Registry backend
  -``` JSON
  +```
  {
  "device": {
  "backend": {
  "type": "registry",
  "config": {
  - "scheme": "https",
  + ...
  + "scheme": "http",
  "host": "my-registry:5000",
  "repo": "test/repo",
  // Base64(username:password)
  - "auth": "<base64_encoded_auth>",
  - "blob_url_scheme": "http"
  + "auth": "<base64_encoded_auth>"
  }
  },
  ...
  