docs: Attempt to configure Netlify build of mitogen.networkgenomics.com

This site has an unknown build config. Specifically I do not know the value of

- Package directory
- Base directory

these are the locations that Netlify looks for netlify.toml. I've already
tried docs/netlify.toml (mitogen-hq#1210), unsuccessfully. This attempt is on the basis
that Package directory == '/', or less likely that Base directory == '/'.

refs mitogen-hq#1209
See
- https://docs.netlify.com/configure-builds/monorepos/#use-a-netlify-configuration-file
- https://docs.netlify.com/configure-builds/file-based-configuration/

netlify.toml

@@ -0,0 +1,2 @@
+[build.environment]
+PYTHON_VERSION = "3.8"