Add manidest tool to index.md

Add Manifest, a tool that enables developer to create easily backends for web applications

Author: SebConejo

collections/productivity-tools/index.md

@@ -41,6 +41,7 @@ items:
  - logdyhq/logdy-core
  - ajayyy/SponsorBlock
  - WofWca/jumpcutter
+ - mnfst/manifest
 display_name: Software productivity tools
 created_by: holman
 ---