Update intro to introduction mechanisms

[mmccool]

Clean up intro to introduction mechanisms, to make clear that:

• Any of the introduction mechanisms can be used
• Multiple introduction mechanisms can be used
• No particular introduction mechanism is mandatory
• The purpose is first contact
• The result is always a URL (NOT TDs or other metadata)
• There might be more than one result and if so they should be used (so in general the result of the introduction process is a SET of URLs pointing to exploration mechanisms).

We might have to clean up some of the text, in particular if it says anywhere an introduction returns a TD. The result should always be a URL, full stop.

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[mmccool]

reviewed; there are indeed a few issues in direct and well-known that talk about fetching TDs that should be cleaned up, but we can do that in a future PR