[Feature Request] Suppress warnings occurred when using custom filter rules

[filzrev]

Is your feature request related to a problem? Please describe.

When using Custom filter rules and run docfx build command.
Following warnings occurred.

Unable to handle following files: filterConfig.yml. Do they miss YamlMime as the first line of file, e.g.: ### YamlMime:ManagedReference?

I knows this warning can be suppressed by explicitly excluding filterConfig.yml from build configs.
But current message seems misleading to fix problem .

Describe the solution you'd like

filter parameter is not passed to SingleDocumentBuilder (Because it is used for docfx metadata command)
So it can't exclude by file path.

I thought, following changes are required to suppress warnings.

1. Add new FilterConfig YAML mime type definition.
   

   docfx/src/Microsoft.DocAsCode.Common/YamlMime.cs

   Lines 10 to 13 in d295256

   	public const string YamlMimePrefix = nameof(YamlMime) + ":";
   	public const string ManagedReference = YamlMimePrefix + nameof(ManagedReference);
   	public const string TableOfContent = YamlMimePrefix + nameof(TableOfContent);
   	public const string XRefMap = YamlMimePrefix + nameof(XRefMap);

2. Suppress warning occurred (by reading YAML mime)
   

   docfx/src/Microsoft.DocAsCode.Build.Engine/SingleDocumentBuilder.cs

   Lines 165 to 177 in 9fe315c

   	var notToHandleItems = files
   	.Where(s => s.Key == null)
   	.SelectMany(s => s)
   	.Where(s => s.Type != DocumentType.Overwrite &&
   	!s.File.EndsWith(".yaml.md", StringComparison.OrdinalIgnoreCase) &&
   	!s.File.EndsWith(".yml.md", StringComparison.OrdinalIgnoreCase))
   	.ToList();
   	if (notToHandleItems.Count > 0)
   	{
   	Logger.LogWarning(
   	$"Unable to handle following files: {notToHandleItems.Select(s => s.File).ToDelimitedString()}. Do they miss `YamlMime` as the first line of file, e.g.: `### YamlMime:ManagedReference`?",
   	code: WarningCodes.Build.UnknownContentType);
   	}

3. Add YAML mime to filterConfig.yml file header.
   ### YamlMime:FilterConfig

[filzrev]

Close this issue.
Because it can be resolved by excluding fileterConfig.yml form build configs.