bpo-37860: re-add netlify.toml to set up deploy previews for docs (#92852)

* Revert "bpo-46184: remove `netlify.toml` (#30272)"

This reverts commit fbaf2e604cd354f1ebc6be029480010c6715a8ca.

* Delete runtime.txt

* Create runtime.txt

* Delete runtime.txt

* Update netlify.toml

* Update netlify.toml

* Add netlify badge

* Update Doc/tools/templates/layout.html

Co-authored-by: Hugo van Kemenade <hugovk@users.noreply.github.com>

* Update layout.html

Co-authored-by: Hugo van Kemenade <hugovk@users.noreply.github.com>