Auto-update API docs version on develop branch

[vgeorge]

In release v2.1.0 we merged develop to main without updating API docs version in next-swagger-doc.json. It is easy to forget to update that file, so I suggest we add a Github Action step to update it automatically. The algorithm would be something like this:

• on commit to develop branch
• run yarn docs:validate (this task auto-update next-swagger-doc.json with package.json version)
• commit file next-swagger-doc.json if changed

cc @kamicut @batpad