docs: support the command systemctl enable apisix-dashboard

[GuoFlight]

Please answer these questions before submitting a pull request, or your PR will get closed.

Why submit this pull request?

• Bugfix
• New feature provided
• Improve performance
• Backport patches

What changes will this PR take into?

Add the following configuration to the document:

[Install]
WantedBy=multi-user.target

Related issues

We cannot use the command systemctl enable apisix-dashboard to configure startup

Test case

echo "[Unit]
Description=apisix-dashboard
Conflicts=apisix-dashboard.service
After=network-online.target

[Service]
WorkingDirectory=/usr/local/apisix-dashboard
ExecStart=/usr/local/apisix-dashboard/manager-api -c /usr/local/apisix-dashboard/conf/conf.yaml

[Install]
WantedBy=multi-user.target" > /usr/lib/systemd/system/apisix-dashboard.service
systemctl daemon-reload
systemctl enable apisix-dashboard

Checklist:

• Did you explain what problem does this PR solve? Or what new features have been added?
• Have you added corresponding test cases?
• Have you modified the corresponding document?
• Is this PR backward compatible? If it is not backward compatible, please discuss on the mailing list first