docs: add versions for enterprise and open-source docs

[dgarcia360]

Prepares JSON files to build enterprise and open-source documentation using JSON data hosted in this repository.

The JSON files will be published at:

• https://docs.scylladb.com/stable/_static/data/opensource_doc_versions.json
• https://docs.scylladb.com/stable/_static/data/enterprise_doc_versions.json

In the future, we can create a new JSON file that follows the same format if enterprise and open-source documentation are integrated into a single repository.

How to test

Check that the JSON files contain all the necessary info to build versioned docs.

Next steps

• Adapt the theme to provide a utility function for reading configuration from external URLs.
• Configure the open-source repository to read from the external URL.
• Configure the enterprise repository to read from the external URL.