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/html_codepreview_test.go

@@ -11,6 +11,7 @@ import (
 
 	"code.gitea.io/gitea/modules/git"
 	"code.gitea.io/gitea/modules/markup"
+	"code.gitea.io/gitea/modules/markup/markdown"
 
 	"github.com/stretchr/testify/assert"
 )
@@ -23,8 +24,8 @@ func TestRenderCodePreview(t *testing.T) {
 	})
 	test := func(input, expected string) {
 		buffer, err := markup.RenderString(&markup.RenderContext{
-			Ctx:  git.DefaultContext,
-			Type: "markdown",
+			Ctx:        git.DefaultContext,
+			MarkupType: markdown.MarkupName,
 		}, input)
 		assert.NoError(t, err)
 		assert.Equal(t, strings.TrimSpace(expected), strings.TrimSpace(buffer))