Currently when a new build of the docs is created, an `index.html` file does not exist. Typically this would be generated from a`docs/README.md` file - which we have - however we're currently using [docs/README.md](https://github.com/matrix-org/synapse/blob/394673055db4df49bfd58c2f6118834a6d928563/docs/README.md) to explain the docs and point to the website. It is not part of the content of the website. So we end up not having an `index.html` file, which will result in a 404 page if one tries to navigate to `https://matrix-org.github.io/synapse/<docs_version>/index.html`.

This isn't a really problem for the default version of the documentation (currently `develop`), as [navigating to the top-level root](https://matrix-org.github.io/synapse/) of the website (without specifying a version) will [redirect](https://github.com/matrix-org/synapse/blob/a77e6925f26597958eccf0ef9956cb13c536e57e/index.html#L2) you to the Welcome and Overview page of the `develop` docs version.

However, ideally once we add a GUI for switching between versions, we'll want to send the user to `matrix-org.github.io/synapse/<version>/index.html`, which currently isn't generated.

This PR modifies the CI that builds the docs to simply copy the rendered [Welcome & Overview page](https://matrix-org.github.io/synapse/develop/welcome_and_overview.html) to `index.html`.