Add project: Posthog

[itz-Amethyst]

What kind of change does this PR introduce?

• [x] Add a project
• [ ] Update a project
• [ ] Remove a project
• [ ] Add or update a category
• [ ] Change configuration
• [ ] Documentation
• [ ] Other, please describe:

Description: posthog : PostHog provides open-source product analytics, session recording, feature flagging and A/B testing that you can self-host.

Checklist:

• [x] I have read the CONTRIBUTING guidelines.
• [x] I have not modified the README.md file. Projects are only supposed to be added or updated within the projects.yaml file since the README.md file is automatically generated.