Versioning for Content Creators section

[nearnshaw]

Add a version selection dropdown for the entire Content creators section.
This will help navigate between the SDK6 and the SDK7 docs. Eventually, as we release more versions with breaking changes or new features, we can also host these as selectable versions in the dropdown.

Considerations and things to research:

• In the future we plan on versioning SDK + Editor + CLI as a bundle with a single version number. That should make this task simpler.
• Can we version only part of the content in the section, and avoid replicating documents that aren't affected by changes? Maybe only what's under the /sdk section, or even only specific pages with changes. That way the content is easier to maintain and better for SEO. This is especially relevant for topics inside "Content creators" that are not tied to the SDK version, like 3D modeling. In case we need to duplicate the content on each version, we'll only actively maintain the newer version.
• When running search, users should only be searching within the version they're currently selecting
• SEO: does it makes sense to unindex older versions? or somehow give more priority to the new ones always
• We should display some kind of note on older versions, warning that the version you're looking at is not the latest, with a link to the latest

[MetaverseUnknower]

On a somewhat related note, the documentation for creating an SDK7 library seems to have been carried over from similar instructions in SDK6 - https://docs.decentraland.org/creator/development-guide/sdk7/create-libraries/

Unlike the original dcl init CLI command, npx @dcl/sdk-commands init does not present any options for what type of project to create, and running it just creates a cube spawner project.