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

models/repo/repo.go

@@ -479,7 +479,6 @@ func (repo *Repository) ComposeMetas(ctx context.Context) map[string]string {
 		metas := map[string]string{
 			"user": repo.OwnerName,
 			"repo": repo.Name,
-			"mode": "comment",
 		}
 
 		unit, err := repo.GetUnit(ctx, unit.TypeExternalTracker)
@@ -521,7 +520,6 @@ func (repo *Repository) ComposeDocumentMetas(ctx context.Context) map[string]str
 		for k, v := range repo.ComposeMetas(ctx) {
 			metas[k] = v
 		}
-		metas["mode"] = "document"
 		repo.DocumentRenderingMetas = metas
 	}
 	return repo.DocumentRenderingMetas