Add link to download public certificate for SPs to validate SAML responses

Changes:
* Add certificate file (public key) in Stub App_Data.
* Add a link in index to download it.

---

Hello there devs!
I wanted to start slow, with this simple issue since I am still learning the basics of git.
Please provide all critics and reviews possible, or even discard this request if the solution is a mess.
Thank you.

I think this could provide more information, maybe something like "The SAML messages sent by the Stub Idp will be signed with [this] certificate. To validate the responses you need to download it and configure your service provider to trust it. *Caution*: Never trust this certificate in a production environment as it will allow anyone to sign in to your application as whatever user they choose."