[Docs] Update mysql-to-doris.md, add scan.startup.mode config to make demo run more smoothly

[maixiaohai]

When not set scan.startup.mode to earliest-offset, will use intial mode，the next part about incrementing data will not work. That's not very friendly to new starters.

[leonardBang]

Thanks for your contribution @maixiaohai ! But the default initial mode will read snapshot data and then read incremental data(i.e. binlog data) automatically, Could you explain more why incremental reading will not work?

[maixiaohai]

Thanks for your contribution @maixiaohai ! But the default initial mode will read snapshot data and then read incremental data(i.e. binlog data) automatically, Could you explain more why incremental reading will not work?

ok. I didn't see error message in cdc task, and I followed the instruction to deploy env. So maybe I should check the source code. It will take some time, and I think this pr could be closed. Thanks for your quick reply~

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