DOC-5279

I don't know much about the secret providers or the rotation mechanism (unless you can point me to the relevant info for RDI) but I've added a starting point for future discussion:

• I've renamed "Deploy a pipeline" to "Set secrets", since most of the page was about the secrets anyway. I've moved the deployment instructions back to the config page for now.
• I've added a skeleton page for external providers (but as I say, I don't know anything about them yet). If there is not going to be enough info to require a separate page, then we could just add this to "Set secrets".
• Another option is to add a new section about authentication. We could maybe have the table of secret names and an overview in the index page and then have separate pages for VM, K8s, and external providers, if that works better.

All input welcome :-)

I'll fix up links from other pages when we're clearer about the structure we want for this. Also, I've got a few changes in mind for the data pipeline config page (including the ones suggested by ChatGPT), but I'll make a separate PR for those.