Add Encryption form to docs

[rarkins]

Describe the proposed change(s).

Renovate's encrypted capability depends on public key cryptography. Now that Mend has deprecated this capability in its hosted app (superseded by a UI-based upload of secrets), it no longer makes sense for it to live at https://app.renovatebot.com/encrypt for users to download/modify.

We should add a page to the Renovate docs which describes how to download/update/use the form for self-hosted use, including generating own public/private key pair. We could generate a brand new pair for the purpose of the docs which users could use to validate that the concept works, before replacing with their own key, but is there a risk that users unknowingly run with the shared key "in production"?

[viceice]

we can validate that this known key can't be used at runtime without an explicit test gpg env?