Explain the distinctions between local installation and Codespaces, and switching between them

[StevenMaude]

We should make it clear in our documentation that users can run locally and/or in Codespaces, at any point of development.

There are some points in the process of developing an OpenSAFELY project where the users might want to migrate from Codespaces to a local installation.

Tasks

1. Ensure that the main tutorial documentation focuses on Codespaces first, with a brief mention that other installation routes are possible, if:
   • your project is not suitable for Codespaces (if we find any following Determine if there are projects where Codespaces might not be suitable #11)
   • you want to work a lot with OpenSAFELY without incurring costs
   • you want to work without a constant internet connection being required
   • you want a more customised environment
2. Explain elsewhere:
   • Any limitations of Codespaces.
   • That the same setup can be used on your own computer, if you have VSCode and Docker installed.
   • It is possible to use an entirely ad-hoc development setup
     • And perhaps that we cannot provide (much? any?) support for this.
3. Move the existing installation guidance to some "For advanced users" section or similar.
4. Run through this guidance with copilots?

Assumptions

That a local dev container setup does work the same as in Codespaces (see #19).