Refactor markup render system (#32612)

This PR removes (almost) all path tricks, and introduces "renderhelper"
package.

Now we can clearly see the rendering behaviors for comment/file/wiki,
more details are in "renderhelper" tests.

Fix #31411 , fix #18592, fix #25632 and maybe more problems. (ps: fix
#32608 by the way)

models/renderhelper/simple_document.go

@@ -0,0 +1,29 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package renderhelper
+
+import (
+	"context"
+
+	"code.gitea.io/gitea/modules/markup"
+)
+
+type SimpleDocument struct {
+	*markup.SimpleRenderHelper
+	ctx      *markup.RenderContext
+	baseLink string
+}
+
+func (r *SimpleDocument) ResolveLink(link string, likeType markup.LinkType) string {
+	return r.ctx.ResolveLinkRelative(r.baseLink, "", link)
+}
+
+var _ markup.RenderHelper = (*SimpleDocument)(nil)
+
+func NewRenderContextSimpleDocument(ctx context.Context, baseLink string) *markup.RenderContext {
+	helper := &SimpleDocument{baseLink: baseLink}
+	rctx := markup.NewRenderContext(ctx).WithHelper(helper).WithMetas(markup.ComposeSimpleDocumentMetas())
+	helper.ctx = rctx
+	return rctx
+}