[Feature Request] Define common metadata for docfx

**Is your feature request related to a problem? Please describe.**
Define `common metadata` for docfx that similar to https://learn.microsoft.com/en-us/contribute/content/metadata.
It's expected to be used by docfx. And other external docfx plugins.

**Describe the solution you'd like**
Add official document about `common metadata for docfx`.

The following are personal suggestions

| Name | Required | Description |
|--------------|----------------------|---------------------------------------------------------------------------------------|
| `title` | Optional | The title of document.
| `description` | Optional | The description of document.
| `date` | Optional | DateTime string that can be parsed with `InvariantCulure` (e.g. 2024-01-31) that will be used for RSS feed(#8820)
| `tags` | Optional | Tag names of document (#9993) or Label names(#177)
| `index` or `order` | Optional | Order of document. It'll be used for automatic TOC generation (#3124)

**Additional context**
If there are any suggestions. I'll modify the above suggestions.