Add dev container setup

[StevenMaude]

Fixes #1047.

This allows someone to work on the docs via Codespaces or in VSCode using the dev containers feature.

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[StevenMaude]

Old discussion; either something with my browser, computer or Codespaces changed, because this problem's now gone.

~This configuration works as is in a local dev container, and makes the port accessible. I'm having real trouble trying to get Codespaces to make the port available. You can curl localhost:8910 and get the response from the MkDocs server, so the server is running.~

~There is this discussion: https://github.com/orgs/community/discussions/61154 which might be related. I'm not sure.~

~I'm similarly having problems even in Codespaces from repositories that I know full well used to work like the research-template and running opensafely jupyter.~

~There's also mention of an new and forthcoming port forwarding mechanism: https://github.com/orgs/community/discussions/57596~

~So maybe return to this in a few weeks and try again. I'd prefer to having it working in Codespaces, and be able to validate that myself, before merging.~

[StevenMaude]

~I should try this again, as I'd had similar problems with the team manual, and that just worked when I tried it with Codespaces more recently.~

It works fine now.