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/console/console.go

@@ -8,7 +8,6 @@ import (
 	"io"
 	"path/filepath"
 	"regexp"
-	"strings"
 
 	"code.gitea.io/gitea/modules/markup"
 	"code.gitea.io/gitea/modules/setting"
@@ -17,9 +16,6 @@ import (
 	"github.com/go-enry/go-enry/v2"
 )
 
-// MarkupName describes markup's name
-var MarkupName = "console"
-
 func init() {
 	markup.RegisterRenderer(Renderer{})
 }
@@ -29,7 +25,7 @@ type Renderer struct{}
 
 // Name implements markup.Renderer
 func (Renderer) Name() string {
-	return MarkupName
+	return "console"
 }
 
 // Extensions implements markup.Renderer
@@ -67,20 +63,3 @@ func (Renderer) Render(ctx *markup.RenderContext, input io.Reader, output io.Wri
 	_, err = output.Write(buf)
 	return err
 }
-
-// Render renders terminal colors to HTML with all specific handling stuff.
-func Render(ctx *markup.RenderContext, input io.Reader, output io.Writer) error {
-	if ctx.Type == "" {
-		ctx.Type = MarkupName
-	}
-	return markup.Render(ctx, input, output)
-}
-
-// RenderString renders terminal colors in string to HTML with all specific handling stuff and return string
-func RenderString(ctx *markup.RenderContext, content string) (string, error) {
-	var buf strings.Builder
-	if err := Render(ctx, strings.NewReader(content), &buf); err != nil {
-		return "", err
-	}
-	return buf.String(), nil
-}