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

modules/markup/markdown/goldmark.go

@@ -72,7 +72,12 @@ func (g *ASTTransformer) Transform(node *ast.Document, reader text.Reader, pc pa
 			g.transformList(ctx, v, rc)
 		case *ast.Text:
 			if v.SoftLineBreak() && !v.HardLineBreak() {
-				if ctx.Metas["mode"] != "document" {
+				// TODO: this was a quite unclear part, old code: `if metas["mode"] != "document" { use comment link break setting }`
+				// many places render non-comment contents with no mode=document, then these contents also use comment's hard line break setting
+				// especially in many tests.
+				if markup.RenderBehaviorForTesting.ForceHardLineBreak {
+					v.SetHardLineBreak(true)
+				} else if ctx.ContentMode == markup.RenderContentAsComment {
 					v.SetHardLineBreak(setting.Markdown.EnableHardLineBreakInComments)
 				} else {
 					v.SetHardLineBreak(setting.Markdown.EnableHardLineBreakInDocuments)