Add using on Read The Docs instructions

docs/using/on_rtd.md

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+# Using on ReadTheDocs
+
+This page describes how to get started with using the MyST parser for documentation generated by and hosted on [Read the Docs](https://readthedocs.org/), which "simplifies software documentation by automating building, versioning, and hosting of your docs for you".
+
+## Example minimal setup
+
+A `.readthedocs.yml` file with this content:
+
+```yaml
+version: 2
+sphinx:
+  configuration: docs/conf.py
+python:
+  version: 3
+  install:
+    - requirements: docs/requirements.txt
+```
+
+```{tip}
+See [Read The Docs' configuration file documentation](https://docs.readthedocs.io/en/stable/config-file/index.html) for more 
+```
+
+A `docs/requirements.txt` with at least this content:
+
+```txt
+myst-parser[sphinx]
+```
+
+A `docs/conf.py` file with at least this content:
+
+```python
+extensions = [
+    'myst_parser',
+]
+
+source_suffix = ['.rst', '.md']
+```
+
+```{tip}
+Use `sphinx-quickstart` to create your `conf.py` file and make the above modifications; see [Getting Started with Sphinx](https://docs.readthedocs.io/en/stable/intro/getting-started-with-sphinx.html#quick-start) for more.  Just remember that to enable markdown-formatted documentation, you're using MyST Parser *instead* of ReCommonMark.  (You still get all of CommonMark, and more!)
+```
+
+And now you're good to go, using MyST for your Sphinx-powered, ReadTheDocs-hosted documentation!
+
+```{seealso}
+- [Read the Docs documentation](https://docs.readthedocs.io/en/stable/index.html)
+- {ref}`The MyST Syntax Guide <using/syntax>`
+```