Implement docs API versioning

[nedsalk]

Our current docs API infrastructure only reflects the master branch (i.e. nightly version). With the merge of #3602, it'll support stable and nightly versions. However, with the introduction of LTS versions, we'll have to start supporting multiple versions of the docs API. This will require changes to either

1. how we interact with the Vercel infrastructure so that we host all of those versions independently, or
2. how we build the docs API so that the hosted application itself can serve multiple versions (e.g. DOCS_API_URL/0.88.0/..., DOCS_API_URL/0.89.0/...).

The related docs-hub issue is

• Support multiple docs versions docs-hub#593