Add example of translated goto function

[NlightNFotis]

Add an example C program and its equivalent goto-functions as produced by cbmc in the
goto-programs folder documentation.

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[owen-mc-diffblue]

Surely you should initialise acc to 0, to make this program make more sense?

[johnnonweiler]

I think this would fit in better together with the goto conversion section http://cprover.diffblue.com/group__goto-programs.html#goto-conversion .
@smowton deleted the placeholder for this example code, so he probably has opinions about this.

[tautschnig]

Of course our documentation will always be up-to-date so that isn't needed :-)

[smowton]

@johnnonweiler @NlightNFotis I have no opinion on this

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 15c6b55).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/99762376

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 1ee9696).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/99784681