Update pgp-key.md

[RubenKelevra]

What I did

— Remove non-working link to keybase.io — Add key ID — Add raw-link to the key file in pgp-keys — Remove not self-signed PGP key in code box (having a single source for the PGP key and the key ID is a bad habit) — Move security reporting link to the bottom

Why I did that

Fixes #1503

To-Do before merge

— A new GitHub repo 'pgp-keys' under lbryio needs to be created — The public key file needs to be signed by the 0x73000EAE82F4283AAF1FCC516CB639B5FFE02E7E key (self-signature) — The signed public key needs to be stored in the master branch of 'pgp-keys' as lbry-key.asc