Update quickstart.md for a better python pdf demo

[qwedc001]

fix issues:
1.getPixmap() function is not recognized,changing to get_pixmap
2.fix TypeError when paddle recognized an empty page
3.pre-stored pageCount to avoid issues
4.added GPU usage

getPixmap好像是个别名函数，我不知道是我配置的问题还是新版本弃用了，在我这里只有get_pixmap能用，可能要麻烦你们再测试一下
然后识别到空页会直接退出弹TypeError有点太难绷了，于是就有了这个pr（

[CLAassistant]

All committers have signed the CLA.

[paddle-bot]

Thanks for your contribution!

[jzhang533]

LGTM
getPixmap is deprecated. see :
https://pymupdf.readthedocs.io/en/latest/znames.html#deprecated-names

[qwedc001]

啊这，突然发现我只更新了中文文档，一会我还会再提个英文文档的修改，麻烦了

[jzhang533]

啊这，突然发现我只更新了中文文档，一会我还会再提个英文文档的修改，麻烦了

feel free to assign me to review. 🚀

[luotao1]

@qwedc001 Thanks for your contribution! You will receive a beautiful PaddlePaddle gift. Please provide your mailing address by filling out the following questionnaire before October 18th.

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[qwedc001]

Thanks @luotao1 much appreciated.

[luotao1]

hi, @qwedc001

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