Update docs workflows to allow multiple versions to be published

[onobc]

Our current docs workflows only allow a single version to be published under https://docs.spring.io/spring-grpc/reference/. This was fine when we had a single version but now that we have GA'ed we hae at least 1.0.0 and 1.0.1-SNAPSHOT.

We need to use the same Antora workflows used by other portfolio projects so that:

1. allow multiple versions of the docs to co-exist
2. not be a "special snowflake" (be consistent w/ other projects)

Note

Also makes sure the search box is present and works.

Take Spring Security for reference:

• there is a docs-build branch
• pay attention to the Antora files in main branch docs directory and the deploy-docs workflow