docs: add JSON fields section to commands with --json flag

[q-soriarty]

Problem

The gh CLI manual has a dedicated "JSON Fields" section listing all available fields for commands that support --json. Our manual only mentions --json in the Options section without listing available fields.

gh CLI (issue list):

### JSON Fields

assignees, author, body, closed, closedAt, comments, createdAt, ...

Copia CLI: No dedicated section — fields only listed in --json flag description.

Fix

Add a "JSON Fields" section to the generated docs for commands that have --json flag. The field list can be extracted from the validJSONFields variable in each command.

Affected commands

All commands with --json support: repo list, repo view, issue list, issue view, pr list, pr view, release list, label list, notification list, search repos, search issues.

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Superseded by #132 (Jekyll documentation migration).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).