Auto-update API docs version on develop branch

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

Start by reading package.json and the existing GitHub Actions workflows, then run yarn docs:validate to observe how next-swagger-doc.json changes from the package version. The work is done when commits to develop trigger the validation and any changed next-swagger-doc.json is committed automatically.

Written by the indexing model from the issue text.