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

routers/web/repo/wiki.go

@@ -289,12 +289,12 @@ func renderViewPage(ctx *context.Context) (*git.Repository, *git.TreeEntry) {
 	}
 
 	rctx := &markup.RenderContext{
-		Ctx:   ctx,
-		Metas: ctx.Repo.Repository.ComposeDocumentMetas(ctx),
+		Ctx:         ctx,
+		ContentMode: markup.RenderContentAsWiki,
+		Metas:       ctx.Repo.Repository.ComposeDocumentMetas(ctx),
 		Links: markup.Links{
 			Base: ctx.Repo.RepoLink,
 		},
-		IsWiki: true,
 	}
 	buf := &strings.Builder{}