Add missing `[limits]` keys to default configuration.

conf/config.example.toml

···
 max-symlink-depth = 16
 update-timeout = "60s"
 max-heap-size-ratio = 0.5 # * RAM_size
+forbidden-domains = []
+# allowed-repository-url-prefixes = <nil>
 
 [observability]
 slow-response-threshold = "500ms"

src/config.go

···
 	// Soft limit on Go heap size, expressed as a fraction of total available RAM.
 	MaxHeapSizeRatio float64 `toml:"max-heap-size-ratio" default:"0.5"`
 	// List of domains unconditionally forbidden for uploads.
-	ForbiddenDomains []string `toml:"forbidden-domains"`
+	ForbiddenDomains []string `toml:"forbidden-domains" default:"[]"`
 	// List of allowed repository URL prefixes. Setting this option prohibits uploading archives.
 	AllowedRepositoryURLPrefixes []string `toml:"allowed-repository-url-prefixes"`
 }