gptel: Improve handling of read only buffers

[pabl0]

• gptel.el (gptel-read-only-response-buffer, gptel--handle-pre-insert): Introduce a new defcustom and don't hard code "LLM response".

• gptel-transient.el (gptel--describe-suffix-send): Inform the user when the source buffer is read-only and clarify that output will be directed to the buffer specified in gptel-read-only-response-buffer. Additionally, remove misleading claims about killing content from a read-only source buffer in "respond in place" mode.

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Not sure if the defcustom is too much, but perhaps it is in the spirit of Emacs to maximal customizability? Also I think "read-only" might be more correct than "read only" but then again English is not my first language?