[docs] add sanitycheck readme

sanitychecks/README.md

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+# TL;DR: Sanity Checks in Kestra
+
+Please check the [full Notion page](https://www.notion.so/kestra-io/Sanity-Checks-17c36907f7b58050ba65efe468e19011?pvs=4) for full information
+
+Sanity checks in Kestra are test flows used to verify functionality of plugins and tasks:
+
+**Structure:**
+
+- Plugin sanity checks: Flow ID is the task name, namespace "sanitychecks", and include Assert tasks to test task outputs.
+- Flow sanity checks: Flow ID mentions main tasks, namespace "sanitychecks", they tests complete workflow (always self-contained)
+
+**External Dependencies:**
+
+- OSS services: Use Docker Run/Stop tasks
+- Cloud services: Leverage sandbox environments when possible
+- SaaS/License services: Not currently supported (or hard coded credentials from demo accounts)
+
+**Organization:**
+
+- All sanity checks are synced to the blueprint repository
+- Plugin checks: Located in plugin repos, pulled to blueprint repo via CI/CD
+- Flow checks: Written directly in blueprint repo into the `sanitychecks/flows` folder. Blueprints with `demo: true` are also synced into `sanitychecks/flows/blueprints`
+
+
+**Deployment:**
+
+For now deploy to preview environment is done via GitSync flows (see Notion page).
+Alternatively, you can upload through UI.