docs: add detailed Long descriptions to commands (gh CLI parity)

[q-soriarty]

Problem

The gh CLI manual has rich content that our commands lack.

Missing detailed Long descriptions

Our Long descriptions are minimal 1-liners. The gh CLI has extended usage explanations with filtering details and documentation links.

Example — gh issue list Long:

List issues in a GitHub repository. By default, this only lists open issues.
The search query syntax is documented here: [link]
On supported GitHub hosts, advanced issue search syntax can be used in the --search query.

copia-cli issue list Long:

List issues in a Copia repository. By default, only open issues are listed.

Should be expanded to document available filtering options (--state, --label, --limit) and their behavior.

Missing parent command examples

Parent command pages (issue, repo, pr) in gh CLI have examples sections showing common workflows. Ours have none.

Missing subcommand categories on parent pages

Resolved in #132 — General/Targeted groups implemented.

Scope

• Expand Long descriptions with filtering details, default behavior, and Copia API references
• Add Example fields to parent commands (issue, repo, pr, label, release, org, search)

Reference

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

[q-soriarty]

Superseded by #132 (Jekyll documentation migration).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

[github-actions]

Closed by #136 (merged into develop).