docs: improve glab alias command docs
Description
This merge request improves the documentation and help text for the glab alias command and its subcommands (set, list, delete).
The changes add detailed descriptions explaining what aliases are (shortcuts for longer commands), provide clearer usage examples with comments, and reorganize the help text to be more user-friendly.
The documentation now includes "Synopsis" sections that explain each command's purpose in plain language, and the examples show practical use cases like creating shortcuts for merge request viewing and setting up shell pipelines.
Related issues and merge requests
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/.