docs: Create a guide on how to migrate license keys from Lemon Squeezy to Polar

[rishi-raj-jain]

trafficstars

• suggest creation of license keys with a given prefix in Polar (say pl-)
• cover a code example of how if the license key input contains pl-, call the polar apis, otherwise use LS apis

(only after https://github.com/polarsource/polar/issues/6566 is resolved)