migrate tea to urfave v3 (#760)

I tested this somewhat, but I haven't been using the cli before so I'm not sure if there are changes - there shouldn't be though.

Reviewed-on: https://gitea.com/gitea/tea/pulls/760
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: TheFox0x7 <thefox0x7@gmail.com>
Co-committed-by: TheFox0x7 <thefox0x7@gmail.com>

docs/docs.go

@@ -0,0 +1,61 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+//go:generates
+package main
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"path/filepath"
+
+	"code.gitea.io/tea/cmd"
+	docs "github.com/urfave/cli-docs/v3"
+	"github.com/urfave/cli/v3"
+)
+
+// CmdDocs generates markdown for tea
+func main() {
+	cli := &cli.Command{
+		Name:        "docs",
+		Hidden:      true,
+		Description: "Generate CLI docs",
+		Action: func(ctx context.Context, c *cli.Command) error {
+
+			md, err := docs.ToMarkdown(cmd.App())
+			if err != nil {
+				return err
+			}
+			outPath := c.String("out")
+			if outPath == "" {
+				fmt.Print(md)
+				return nil
+			}
+
+			if err := os.MkdirAll(filepath.Dir(outPath), os.ModePerm); err != nil {
+				return err
+			}
+
+			fi, err := os.Create(outPath)
+			if err != nil {
+				return err
+			}
+			defer fi.Close()
+			if _, err := fi.WriteString(md); err != nil {
+				return err
+			}
+
+			return nil
+
+		},
+		Flags: []cli.Flag{
+			&cli.StringFlag{
+				Name:    "out",
+				Usage:   "Path to output docs to, otherwise prints to stdout",
+				Aliases: []string{"o"},
+			},
+		},
+	}
+	cli.Run(context.Background(), os.Args)
+}