docs: add info on deprecation messages

Description

This change updates the contributing guidelines for the GitLab CLI project. It clarifies the name of the documentation style guide by adding "GitLab CLI (glab)" to make it more specific.

Additionally, it adds a new section about deprecation messages, which provides guidance on how to properly format warning messages when features are being phased out or removed from the tool.

Related issues and merge requests

• Add deprecation guidance to glab style guide (gitlab!234955 - merged)

Documentation

If this MR adds or updates CLI command documentation, follow the conventions in the GitLab CLI (glab) documentation style guide, run make gen-docs, and commit the updated files in docs/source/.