Added documentation

Author: jcpitre

functions-python/tasks_executor/src/tasks/pmtiles_builder/README.md

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+# Build pmtile for a specific GTFS dataset
+
+This task generates the pmtiles for a provided dataset.
+pmtiles are used for displaying routes and stops in the UI
+
+## Task ID
+Use task Id: `build_pmtiles`
+
+## Usage
+The function receive the following payload:
+```
+   {
+      "feed_stable_id": str, 
+      "dataset_stable_id*: str 
+   }
+```
+
+Example:
+```json
+   {
+      "feed_stable_id": "mdb-1004", 
+      "dataset_stable_id": "mdb-1004-202507081807"
+   }
+```
+
+The task will verify that the dataset stable id starts with the feed stable id (mdb-1004 in our example)
+
+# GCP environment variables
+The function uses the following environment variables:
+- `ENV`: The environment to use. It can be `dev`, `staging` or `prod`. Default is `dev`.
+- `DATASETS_BUCKET_NAME`: The bucket name where the datasets are stored. The task will fail if this is not defined.  The variable has to include the suffix, like `-dev`, `-qa` or `-prod`.