doc: Improvements to Reproduce section

* Increase the required build host storage to 200GiB
* Add a note about FVP not being cycle accurate
* Remove intructions about launching telnet manually to connect to the SI
  terminal

Issue-Id: SCM-5314
Signed-off-by: Diego Sueiro <diego.sueiro@arm.com>
Change-Id: I42916ae0c2233280cc52ea231bff5cad6b6a9d18