Add Codespaces to the team manual

Create internal documentation in the team manual to:

[x] Point to the OpenSAFELY docs page(s) on Codespaces and explain how we expect Codespaces to be used
[x] Record our policy set in the OpenSAFELY GitHub organisation and explain the reasoning for the settings (see https://github.com/opensafely-core/codespaces-initiative/issues/5#issuecomment-2037714770)
[x] Explain what to do if the policy is too strict
[x] Record our reasoning for the monthly spend limit (see ticket)
[x] Explain what to do if the spend limit is reached
[x] Reminder about our usage policy (a link to the Policies for Researchers document)
[x] Explain what to do if something is uploaded to a Codespace that shouldn’t be

Also,

[x] Add a section in the tech support routing table to point Codespaces requests to Team REX.

opensafely-core / codespaces-initiative