docs: improve help text for glab cluster commands

Description

Improves the help text across all glab cluster commands and subcommands for consistency, clarity, and accuracy. Changes follow the GitLab CLI documentation style guide.

Changes

• New Long descriptions added to parent commands (cluster, agent, token, token-cache) so that --help output provides useful context rather than falling back to the Short description.
• Short descriptions updated to use imperative mood and end with a period, consistent with the style guide.
• Long descriptions** rewritten across subcommands to add distinct value beyond what Short already says, remove markdown headers and lists that render as raw text in the terminal, and fix inaccuracies.
• Flag descriptions updated across graph, update-kubeconfig, check-manifest-usage, and bootstrap to use single quotes for inline code references, end with a period, and remove redundant (default false) annotations.
• Example fields added to agent list, token list, token revoke, token-cache clear, and token-cache list, which previously had none. The graph examples are inlined from the deleted example.md file.

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/.