chore: import upstream snapshot with attribution

docs/workflow/schedule.md

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+# Schedule
+
+![schedule](../images/schedule.png#only-light)
+![schedule](../images/schedule-dark.png#only-dark)
+
+Workflows can run on a repeating basis with schedules. This is suitable in cases where a workflow is run against a dynamically expanding input, like an API service or directory of files. 
+
+The schedule method takes a cron expression, list of static elements (which dynamically expand i.e. API service, directory listing) and an optional maximum number of iterations.
+
+Below are a couple example cron expressions.
+
+```bash
+# ┌─────────────── minute (0 - 59)
+# | ┌───────────── hour (0 - 23)
+# | | ┌─────────── day of the month (1 - 31)
+# | | | ┌───────── month (1 - 12)
+# | | | | ┌─────── day of the week (0 - 6)
+# | | | | | ┌───── second (0 - 59)
+# | | | | | |
+  * * * * * *      # Run every second
+0/5 * * * *        # Run every 5 minutes
+  0 0 1 * *        # Run monthly on 1st
+  0 0 1 1 *        # Run on Jan 1 at 12am
+  0 0 * * mon,wed  # Run Monday and Wednesday
+```
+
+## Python
+Simple workflow [scheduled](../#txtai.workflow.base.Workflow.schedule) with Python.
+
+```python
+workflow = Workflow(tasks)
+workflow.schedule("0/5 * * * *", elements)
+```
+
+See the link below for a more detailed example.
+
+| Notebook  | Description  |       |
+|:----------|:-------------|------:|
+| [Workflow Scheduling](https://github.com/neuml/txtai/blob/master/examples/27_Workflow_scheduling.ipynb) | Schedule workflows with cron expressions | [![Open In Colab](https://colab.research.google.com/assets/colab-badge.svg)](https://colab.research.google.com/github/neuml/txtai/blob/master/examples/27_Workflow_scheduling.ipynb) |
+
+## Configuration 
+Simple workflow scheduled with configuration.
+
+```yaml
+workflow:
+  index:
+    schedule:
+      cron: 0/5 * * * *
+      elements: [...]
+    tasks: [...]
+```
+
+```python
+# Create and run the workflow
+from txtai import Application
+
+# Create and run the workflow
+app = Application("workflow.yml")
+
+# Wait for scheduled workflows
+app.wait()
+```
+
+See the links below for more information on cron expressions.
+
+- [cron overview](https://en.wikipedia.org/wiki/Cron)
+- [croniter - library used by txtai](https://github.com/kiorky/croniter)