docs(chat): add creating a chat session doc

Author: Andrew-Lahikainen

docs/features/talk/creating_a_chat_session.md

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+# Creating a Chat Session
+
+## Portal
+
+Portal users can create [chat sessions](../../glossary/c/chat_session.md) in 2 places:
+* the Request Support screen (pre-ticket chat)
+* on an existing ticket (post-ticket chat)
+
+### Pre-ticket chat
+
+Portal users can now request to chat before they have created a ticket. They can do this by heading to `Request Support > Chat to a Technician`. Users can only have one active (in [waiting status](../../glossary/c/chat_session.md)) pre-ticket chat open at any one time.
+
+### Post-ticket chat
+
+Users can also chat on an existing ticket by heading to `Open Tickets > {ticket} > Chat Icon`. These chat sessions will have the ticket number attached to them so members/resources will be able to easily pull up the relevant ticket. Post-ticket chat sessions cannot be created on closed tickets.
+
+## Tech
+
+Tech users can only create chat sessions from a ticket at this stage and only if that ticket has an online contact assigned to the ticket. This is what's known as a push-chat, where a chat session is created with a target user.
+