add period over period example in docs

[jpetey75]

details here https://lightdash.slack.com/archives/C047XUL7QAZ/p1743508919778609?thread_ts=1740049133.722679&cid=C047XUL7QAZ

example video: https://www.loom.com/share/190e774abebb48179855ba62f18b4895

also give example with cumulative metrics using table calcs

and a monthly example in addition to the year example in the loom

month over month example like this: https://demo.lightdash.com/share/3znkJz0IIEQ4loF0Yc4F8

maybe also an option to do this by joining the same table twice to a single date spine? need to experiment with that

[hamzahc1]

@jpetey75 can we publish these this week?