[PLUGINS] Add field help text for `sitemap_gen` metadata

[utksh1]

Summary

plugins/sitemap_gen/metadata.json currently loads into the generated UI, but both fields lack help guidance.

Why this matters

SecuScan renders plugin forms from metadata. Missing help text forces contributors and users to guess at valid inputs, especially for tools with sensitive or specialized parameters.

Scope

• Add concise help text for each exposed field in plugins/sitemap_gen/metadata.json.
• Keep the copy specific to the underlying tool behavior.
• Refresh the plugin checksum after editing metadata.

Acceptance criteria

• Every user-facing field in plugins/sitemap_gen/metadata.json has meaningful help copy.
• The copy explains expected input format or operator intent.
• Plugin checksum is refreshed.
• The plugin still passes validation and loads in the UI.

[anshul23102]

Interested in contributing under NSoC'26. This is a valuable issue. Happy to work on it.

/assign

[Srishti-Gupta74]

Hi @utksh1!
I'd be happy to work on this issue under GSSoC '26. I plan to improve the plugin metadata by adding clear guidance for each exposed field, ensuring users can easily understand the expected inputs. I'll also update the checksum and confirm that the plugin continues to load and validate successfully. Kindly assign it to me. Thank you.