Add versioning and version selection to Git Proxy documentation

Git Proxy v2 extended Git Proxy significantly over v1, which is likely to remain in use for some time. The project should retain v1 documentation when making v2 docs available.

This will need to be completed to release documentation for the v2 release.

**Describe the solution you'd like**

Enable the Docusaurus versioning features, add version selection to the navigation and create scripts (and guide content) to create a new documentation versions. See https://docusaurus.io/docs/versioning for details on versioning.

**Describe alternatives you've considered**

Set up a separate deployment of the docs site for v1 documentation. However, this will make discovery harder (will require cross-linking) and will require FINOS to create an additional deployment, which will be hard to maintain from the project's repository.

**Additional context**

As a software project,
I recommend NOT creating a version for each minor release, rather we should create one at major releases only and then add to it as features are introduced. However a pattern will need to be established for marking which minor version new features were added in (e.g. since v2.1.x) in both the config reference and any docs for a feature.