Resolves #163 "Create service definition for talks.cam"

IMO all of the suitable for being publicly-visible content is present; the page links to a private GitLab project README.md. See commit message 2d26b4cd for why I put it under "Generic" in the top-level Guidebook tree.

I would have liked to have got the private docs closer to being finished but in the light of the recent incident, I decided that publishing this sooner rather than later would be better for the benefit of all UIS staff, with only the most important info present in the private docs: namely "please keep Adam and Raymond in the loop, please note only infra-sas have privileged access".

NB I added the Infra SaS group to the Members of the parent Legacy Talks.cam project, hopefully this has done the right thing permissions-wise to let them see the Documentation sub-project.

Anyhow, we can create a new issue to finish off the private docs.