Include `systemd-timesyncd` into the list of NTP services

Change Request

This repository is ONLY used to solve problems related to DOCS-CN.
For other issues (TiDB, TiKV, etc), please move to other repositories.

Please answer the following questions before submitting your issue. Thanks!

1. Describe what you find is inappropriate or missing in the existing docs.

In https://docs.pingcap.com/zh/tidb/dev/check-before-deployment#%E6%A3%80%E6%B5%8B%E5%8F%8A%E5%AE%89%E8%A3%85-ntp-%E6%9C%8D%E5%8A%A1, the doc only suggested checking against ntpd and chronyd.

However, newer distros such as Ubuntu 20.04 have switched to systemd's built-in systemd-timesyncd already, so a user blindly following this instruction will mistakenly install an unnecessary piece of service.

2. Describe your suggestion or addition.

• Separate ntpd, chronyd and systemd-timesyncd instructions into 3 tabs
• In the systemd-timesyncd tab,
  • check existence using sudo systemctl status systemd-timesyncd
  • check synchronization status using timedatectl, output should contain

System clock synchronized: yes
              NTP service: active

3. Provide some reference materials (documents, websites, etc) if you could.

https://unix.stackexchange.com/questions/504381/chrony-vs-systemd-timesyncd-what-are-the-differences-and-use-cases-as-ntp-cli/504385#504385

https://www.digitalocean.com/community/tutorials/how-to-set-up-time-synchronization-on-ubuntu-20-04

Start with the linked TiDB deployment documentation section on checking and installing NTP services. Separate the existing ntpd and chronyd guidance into tabs and add a systemd-timesyncd tab using the requested systemctl and timedatectl checks. Done means the page covers all three services without directing Ubuntu 20.04 users to install an unnecessary service.