Add Featurevisor

[fahad19]

What's done

• New section created for "Feature management"
• Added Featurevisor open source project under it: https://featurevisor.com

What is Featurevisor?

• A Git-based feature management tool
• Make changes via Pull Requests
• Merging PRs triggers CI/CD pipeline:
  • building datafiles (JSON files): https://featurevisor.com/docs/building-datafiles/
  • and uploading them to your desired destination: example with Cloudflare https://featurevisor.com/docs/integrations/cloudflare-pages/
• Fetch and consume datafiles and evaluate feature flag values using PHP SDK: https://featurevisor.com/docs/sdks/php/

Quick start guide available here: https://featurevisor.com/docs/quick-start/

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I am quite unsure if listing a free open source project I created is considered self-promotion. much easier to be seen that way if I was sharing a book, podcast, video, etc.

Fully understand if this PR is not accepted.