Add Rail402 to the community x402 facilitators - #3116

hey there, @tolgayayci!

thanks for adding your facilitator to our docs!! i've left some suggestions inline, with some changes. i'm not trying to say it must be committed like this, but here are the things i'm hoping to achieve here:

1. i'd like for Rail402 to follow (roughly) the same pattern as the other community-run facilitator descriptions. so, no license mention, and linking only to the source repo and documentation. the endpoint URLs, playground, etc. can all be found from your docs.
2. i've dropped the upto addition. it's running through your own (unaudited) settlement contract, which isn't currently part of the x402 spec. since the trust model is so different from exact, i'm hesitant to include that detail here. if upto does become a part of x402, i'd be happy to add it here in a follow-up PR.

thanks again! let me know if you have any questions!