AVRO-3867: [Python][Build] Fix the broken link to the Python API doc in the web site

[sarutak]

AVRO-3867

What is the purpose of the change

This PR fixes an issue that the link to the Python API doc in the web site is broken. I built the website by ./build.sh dist and then tried to access to the Python API doc but the link seems broken.

The cause is py/build.sh dist doesn't generate the document. So this PR includes a fix to generate the document when we run py/build.sh dist.

Verifying this change

Built the web site and confirmed the link is available.

Documentation

• Does this pull request introduce a new feature? (no)