Add intro blurb to the Go static site generated

[sacOO7]

To standardise all SDK docs and help customers understand the difference between auto-generated docs and other API reference docs available on the main site, we’ve worked on a generic piece of text to include on the intro page of all SDKs. As part of this ticket, the following text needs to be added to the intro page of the Go SDK.

# Ably Go Client Library SDK API Reference

The Go Client Library SDK supports a realtime and a REST interface. The Go API references are generated from the [Ably Go Client Library SDK source code](https://github.com/ably/ably-go/) using [godoc](https://pkg.go.dev/golang.org/x/tools/cmd/godoc) and structured by classes.

The realtime interface enables a client to maintain a persistent connection to Ably and publish, subscribe and be present on channels. The REST interface is stateless and typically implemented server-side. It is used to make requests such as retrieving statistics, token authentication and publishing to a channel.

View the [Ably docs](https://ably.com/docs/) for conceptual information on using Ably, and for API references featuring all languages. The combined [API references](https://ably.com/docs/api/) are organized by features and split between the [realtime](https://ably.com/docs/api/realtime-sdk) and [REST](https://ably.com/docs/api/rest-sdk) interfaces.

[sync-by-unito]

➤ Automation for Jira commented:

The link to the corresponding Jira issue is https://ably.atlassian.net/browse/SDK-2947