Add polar.sh to our E-commerce guide

[sarah11918]

Polar bills themselves as an open-source, Lemon Squeezy/Paddle alternative. Especially now that they have an Astro integration, this could be a good addition to our e-commerce page.

We don't need a whole guide, but just like we mention Lemon Squeezy, could be good to mention and link to https://docs.polar.sh/documentation/adapters/polar-shastro

[caesiumtea]

As someone who was just browsing the e-commerce page the other day, I would have loved to know about an open source option with an Astro integration!

[sarah11918]

Sounds like a +1 from you! 😄