Zhongshan to Hong Kong Airport

I can see the Air India is on the list and Indigo is not, so if you hold a flight ticket of Air India departing from HK airport, then you can buy the ferry ticket from Zhognshan to Skypier and you can buy the ticket on cksp.com.hk
If you take Indigo, then you can take a cross-border bus (ensure you are eligible for entry to HK as by this way you need to enter HK, not a transit in HK) at Pal Spring West Area (中山棕榈彩虹（西区站), the Summit Hotel (中山盛景尚峰酒店永东东区站) or 中山雅居乐世紀优活商业广场(沙溪站).