Fix docs custom stable version

[albertvillanova]

Currently docs default version is 1.5.0. This PR fixes this and sets the latest version instead.

[albertvillanova]

I see that @lhoestq overlooked this PR with his commit 07e2b05. 😢

I'm adding a script so that this issue does not happen again.

[albertvillanova]

For the moment, the script only includes update_custom_js, but in a follow-up PR I will include all the required steps to make a package release.

[lhoestq]

I think we just need to clarify the release process in setup.py instead of adding a script that does the replacement

[albertvillanova]

@lhoestq I really think we should implement a script that performs the release (instead of doing it manually as it is done now), as it is already the case in transformers. I will do it in a next PR.

For the moment, this PR includes one of the steps of the release script.

[lhoestq]

Let's automate this then :)