FMod - Help menu adjustments

In keeping with the next Abiathar version's theme of being more helpful to new users, I recently reviewed and updated the contents of the Help menu. The GitHub issue tracker was never used and isn't as convenient as just posting to the forum, so the Report Bug item has been removed. The PCKF Thread link was still pointing to the temporary domain for the forum, so I changed it to now go to the classic one. The manual PDF is extremely out of date, so the Documentation link has been replaced with a link to the KeenWiki page on Abiathar, which has a brief intro to the editor.