docs(schedule): add synopsis and examples to schedule commands

Description

Adds Long (Synopsis) fields and improved Examples to the schedule command and its five subcommands, as part of the style guide audit identified in #8263 (closed).

Each Synopsis explains the command's default behavior, key flags, and when to use --repo to target another project. The Examples section on each page is annotated with # comment lines that describe the intent of each invocation; output-only example lines from the previous heredocs have been dropped in favor of clean command examples.

Commands updated:

• schedule
• schedule create
• schedule delete
• schedule list
• schedule run
• schedule update

Content follows the documentation style guide and the GitLab CLI (glab) documentation style guide.

Related issues and merge requests

• docs: Fix CLI documentation inconsistencies ide... (#8263 - closed)

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/.