Allow pinning the theme version in docs.json

[novusnota]

Currently, there is a problem with custom CSS — while allowing easy fixes and adjustments for common Mintlify identifiers, it is not reliable due to the possibility of backwards-incompatible changes (or any changes, really) of the theme. Thus, any CSS customization is fragile and error-prone, as a single update to the Mintlify theme on Mintlify's side could break it.

One can only set the theme in docs.json with the "theme" property, but they cannot pin a concrete version of the specified theme. Allowing users to do so will resolve such issues.