docs: add aliases section to commands with short forms

[q-soriarty]

Problem

Some commands have aliases (e.g. list has ls). The gh CLI manual shows an "Aliases" section for these commands. Our manual doesn't.

gh CLI:

### Aliases

gh issue ls

Fix

The gen-docs.go post-processor should detect commands with aliases and add an "Aliases" section showing the short forms.

Affected commands

Commands with Aliases: []string{"ls"} or similar in their Cobra definition.

Reference

https://cli.github.com/manual/gh_issue_list

[q-soriarty]

Superseded by #132 (Jekyll documentation migration). Will be closed when #132 is merged.

[github-actions]

Closed by #136 (merged into develop).