Add tutorials with other techniques

[nathanshammah]

Issue Description

Right now, we only have tutorials on ZNE under https://mitiq.readthedocs.io/en/latest/examples/examples.html. It would be great to add tutorials on PEC, CDR, REM, etc.

[cryotheta]

Hi, I can do it for CDR.

[nathanshammah]

Thanks @cryotheta, that'd be fantastic! Here are some guidelines for contributing examples to the docs. Here some more general guidelines about contributing to Mitiq with a pull request. Hope this helps and feel free to drop in at any of the Mitiq community calls we host at 6pm CEST / 12pm EST on Fridays (including today), on the Unitary Fund Discord server, in the #community-call channel, http://discord.unitary.fund/.