Pin npm version for doc building

[fxmarty]

cc @echarlaix, npm 10.0.0 released 7 days ago apparently breaks this step:

 ERR! code EBADENGINE
1.096 npm ERR! engine Unsupported engine
1.096 npm ERR! engine Not compatible with your version of node/npm: [email protected]
1.097 npm ERR! notsup Not compatible with your version of node/npm: [email protected]
1.097 npm ERR! notsup Required: {"node":"^18.17.0 || >=20.5.0"}
1.097 npm ERR! notsup Actual:   {"npm":"9.2.0"}
1.099 
1.099 npm ERR! A complete log of this run can be found in:
1.099 npm ERR!     /root/.npm/_logs/2023-09-07T08_57_22_510Z-debug-0.log

[HuggingFaceDocBuilderDev]

The documentation is not available anymore as the PR was closed or merged.

[echarlaix]

Thanks a lot @fxmarty !!