Assistant: update docs on custom chat modes

[sharon-wang]

We recently added docs on Custom Chat modes: https://positron.posit.co/assistant.html#custom-chat-modes

However, with the 1.106 merge, this functionality will be known as "Custom Agents", and we'll need to update our docs accordingly.

We'll likely need to update

• where the custom agents files go (currently go in a chatmodes directory)
• the file names for the custom agents files (currently "chatmode" is in the file name)
• any changes to functionality