Website: add draft documentation for failure handling

The documentation isn't complete yet, I want to have a nice flow diagram
in there as well. But it's a start.

web/project-website/content/usage/failure-handling/_index.md

@@ -0,0 +1,28 @@
+---
+title: Failure Handling
+draft: true
+---
+
+Flamenco has a few different approaches to try and complete a job, even when
+workers sometimes fail at certain tasks. These are the thoughts behind its fault
+handling design:
+
+- When a worker fails a task, it **might be a problem with the worker** (maybe
+  not enough memory to render this shot) or it may be a problem with the blend
+  file it's rendering.
+- **A worker only gets to fail at a task once**, so if it fails the task it
+  won't get that one again, and the task is marked `soft-failed`. If you have
+  multiple workers, then up to 3 workers (by default, see
+  task_fail_after_softfail_count: 3` in the `flamenco-manager.yaml`) can have a
+  go at the task before the task is really considered `failed`.
+- When a worker fails multiple tasks of a job (by default also 3, see
+  `blocklist_threshold: 3` in `flamenco-manager.yaml`) it gets blocklisted and
+  is no longer allowed to do tasks of that type for that job.
+
+There are two reasons a job may fail:
+
+1. If there are too **many failed tasks**, the job will fail.
+2. If there is **no worker left** to complete the job, the job will fail. Note
+   that this also considers offline/asleep workers as "available", as they may
+   come online at any moment. Or maybe that'll never happen, but that's not
+   something Flamenco knows. Be sure to delete stale workers.