Upgrade and align with Docsy's latest release

[chalin]

There have been a few successful migration and/or Docsy upgrade efforts in 2024. I think that it might be time to upgrade gRPC's website too, if only to ensure that the site isn't using 3rd-party libraries that have reach EOL (namely, Bootstrap and Font Awesome). I'm not sure how old Docsy is here.

A notable such effort is for the Kubernetes website. For details about the K8s effort and others, see the "Website improvements for other CNCF projects - upgrades to using Docsy" section of cncf/techdocs#227.

We might also want to consider migrating from https://github.com/grpc/grpc.io-docsy.git to the base Docsy repo.

/cc @ejona86 @nate-double-u - thoughts?

[temawi]

@sreenithi FYI