Add a drop down button that allows users to sift through older and newer versions of the documentation.

[DMills27]

Description

For various reasons, it might be necessary to have an older reference to the documentation available. To accommodate this need, we can create Git tags, in the documentation website, for different major releases of sigstore's constituent repositories and link the version of that documentation to that specific branch. This will allow developers to easily access documentation related to specific releases of the software, even if newer versions have been released.

This can be implemented fairly easily by following the example set by this site (as in the image shown below), which has also used Git tags to organise documentation by release version. By creating Git tags in the documentation site for major releases of sigstore repositories, we can ensure that documentation is easily accessible and that developers have a clear understanding of how the software has evolved over time. This will allow developers to easily navigate to the specific release version they need and access the appropriate documentation.