Add /.well-known/security.txt (RFC 9116) (#1192)

* Add /.well-known/security.txt (RFC 9116)

etcd.io serves no security.txt at the well-known location, so a researcher
following RFC 9116 has no machine-discoverable pointer to etcd's security
reporting process. Added under static/, pointing at the project's GitHub
Security Advisory intake and security policy.

Signed-off-by: Kobi Hikri <kobi.hikri@gmail.com>

* Apply suggestion from @jberkus

---------

Signed-off-by: Kobi Hikri <kobi.hikri@gmail.com>
Co-authored-by: Josh Berkus <josh@agliodbs.com>

Author: kobihikri

static/.well-known/security.txt

@@ -0,0 +1,5 @@
+Contact: https://github.com/etcd-io/etcd/security/advisories/new
+Expires: 2031-07-14T00:00:00.000Z
+Preferred-Languages: en
+Canonical: https://etcd.io/.well-known/security.txt
+Policy: https://github.com/etcd-io/etcd/blob/main/security/README.md