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/misc/markup.go

@@ -6,6 +6,7 @@ package misc
 
 import (
 	api "code.gitea.io/gitea/modules/structs"
+	"code.gitea.io/gitea/modules/util"
 	"code.gitea.io/gitea/modules/web"
 	"code.gitea.io/gitea/routers/common"
 	"code.gitea.io/gitea/services/context"
@@ -14,5 +15,6 @@ import (
 // Markup render markup document to HTML
 func Markup(ctx *context.Context) {
 	form := web.GetForm(ctx).(*api.MarkupOption)
-	common.RenderMarkup(ctx.Base, ctx.Repo, form.Mode, form.Text, form.Context, form.FilePath, form.Wiki)
+	mode := util.Iif(form.Wiki, "wiki", form.Mode) //nolint:staticcheck
+	common.RenderMarkup(ctx.Base, ctx.Repo, mode, form.Text, form.Context, form.FilePath)
 }