Fix docs custom stable version
Currently docs default version is 1.5.0. This PR fixes this and sets the latest version instead.
I see that @lhoestq overlooked this PR with his commit 07e2b05. 😢 

I'm adding a script so that this issue does not happen again.
 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. I think we just need to clarify the release process in setup.py instead of adding a script that does the replacement @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.