Refactor render system (#32492)

There were too many patches to the Render system, it's really difficult
to make further improvements.

This PR clears the legacy problems and fix TODOs.

1. Rename `RenderContext.Type` to `RenderContext.MarkupType` to clarify
its usage.
2. Use `ContentMode` to replace `meta["mode"]` and `IsWiki`, to clarify
the rendering behaviors.
3. Use "wiki" mode instead of "mode=gfm + wiki=true"
4. Merge `renderByType` and `renderByFile`
5. Add more comments

----

The problem of "mode=document": in many cases it is not set, so many
non-comment places use comment's hard line break incorrectly

templates/swagger/v1_json.tmpl

@@ -22615,7 +22615,7 @@
           "type": "string"
         },
         "Mode": {
-          "description": "Mode to render (comment, gfm, markdown)\n\nin: body",
+          "description": "Mode to render (markdown, comment, wiki, file)\n\nin: body",
           "type": "string"
         },
         "Text": {
@@ -22623,7 +22623,7 @@
           "type": "string"
         },
         "Wiki": {
-          "description": "Is it a wiki page ?\n\nin: body",
+          "description": "Is it a wiki page? (use mode=wiki instead)\n\nDeprecated: true\nin: body",
           "type": "boolean"
         }
       },
@@ -22642,7 +22642,7 @@
           "type": "string"
         },
         "Mode": {
-          "description": "Mode to render (comment, gfm, markdown, file)\n\nin: body",
+          "description": "Mode to render (markdown, comment, wiki, file)\n\nin: body",
           "type": "string"
         },
         "Text": {
@@ -22650,7 +22650,7 @@
           "type": "string"
         },
         "Wiki": {
-          "description": "Is it a wiki page ?\n\nin: body",
+          "description": "Is it a wiki page? (use mode=wiki instead)\n\nDeprecated: true\nin: body",
           "type": "boolean"
         }
       },