ci: Save the API description with each docs version

Context

Dagger's documentation website has current docs and saved docs for older versions.

The API reference pages read an API description file during the website build. This file contains the API types, fields, and text shown on those pages.

Problem

All docs versions read the current API description. This can break older pages when the current API changes.

For example, on #13992:

1. A saved beta page referred to ModuleManifestV1.
2. The current API description no longer contained that type.
3. The page could not be built, so the docs build failed in CI.

dagger generate docs updates the current pages only. It does not repair the saved beta pages.

Solution

When the docs module saves a docs version, also save its API description. Both must come from the same Git revision.

During the website build, each docs version must read its own saved API description. Current pages continue to use the current description.

Add the matching API descriptions for existing docs versions as part of this change.