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