Remove section about parallel training with Ray (#12770)

The Ray integration is currently broken, having these docs around
suggest that this functionality is currently available.

website/docs/usage/training.mdx

@@ -11,7 +11,6 @@ menu:
  - ['Custom Functions', 'custom-functions']
  - ['Initialization', 'initialization']
  - ['Data Utilities', 'data']
- - ['Parallel Training', 'parallel-training']
  - ['Internal API', 'api']
 ---
 
@@ -1565,77 +1564,6 @@ token-based annotations like the dependency parse or entity labels, you'll need
 to take care to adjust the `Example` object so its annotations match and remain
 valid.
 
-## Parallel & distributed training with Ray {id="parallel-training"}
-
-> #### Installation
->
-> ```bash
-> $ pip install -U %%SPACY_PKG_NAME[ray]%%SPACY_PKG_FLAGS
-> # Check that the CLI is registered
-> $ python -m spacy ray --help
-> ```
-
-[Ray](https://ray.io/) is a fast and simple framework for building and running
-**distributed applications**. You can use Ray to train spaCy on one or more
-remote machines, potentially speeding up your training process. Parallel
-training won't always be faster though – it depends on your batch size, models,
-and hardware.
-
-<Infobox variant="warning">
-
-To use Ray with spaCy, you need the
-[`spacy-ray`](https://github.com/explosion/spacy-ray) package installed.
-Installing the package will automatically add the `ray` command to the spaCy
-CLI.
-
-</Infobox>
-
-The [`spacy ray train`](/api/cli#ray-train) command follows the same API as
-[`spacy train`](/api/cli#train), with a few extra options to configure the Ray
-setup. You can optionally set the `--address` option to point to your Ray
-cluster. If it's not set, Ray will run locally.
-
-```bash
-python -m spacy ray train config.cfg --n-workers 2
-```
-
-<Project id="integrations/ray">
-
-Get started with parallel training using our project template. It trains a
-simple model on a Universal Dependencies Treebank and lets you parallelize the
-training with Ray.
-
-</Project>
-
-### How parallel training works {id="parallel-training-details"}
-
-Each worker receives a shard of the **data** and builds a copy of the **model
-and optimizer** from the [`config.cfg`](#config). It also has a communication
-channel to **pass gradients and parameters** to the other workers. Additionally,
-each worker is given ownership of a subset of the parameter arrays. Every
-parameter array is owned by exactly one worker, and the workers are given a
-mapping so they know which worker owns which parameter.
-
-![Illustration of setup](/images/spacy-ray.svg)
-
-As training proceeds, every worker will be computing gradients for **all** of
-the model parameters. When they compute gradients for parameters they don't own,
-they'll **send them to the worker** that does own that parameter, along with a
-version identifier so that the owner can decide whether to discard the gradient.
-Workers use the gradients they receive and the ones they compute locally to
-update the parameters they own, and then broadcast the updated array and a new
-version ID to the other workers.
-
-This training procedure is **asynchronous** and **non-blocking**. Workers always
-push their gradient increments and parameter updates, they do not have to pull
-them and block on the result, so the transfers can happen in the background,
-overlapped with the actual training work. The workers also do not have to stop
-and wait for each other ("synchronize") at the start of each batch. This is very
-useful for spaCy, because spaCy is often trained on long documents, which means
-**batches can vary in size** significantly. Uneven workloads make synchronous
-gradient descent inefficient, because if one batch is slow, all of the other
-workers are stuck waiting for it to complete before they can continue.
-
 ## Internal training API {id="api"}
 
 <Infobox variant="danger">