Update Synology guide

[kenjenkins]

Page: https://www.pomerium.com/docs/guides/synology

What's incorrect or missing

The recommended configuration may not work, starting with Pomerium v0.21. Sign-in redirects may be too large for nginx (which is used by DSM's Reverse Proxy) in its default configuration.

What's the resolution?

Rework the guide instructions to set up port forwarding to Pomerium directly, without the extra DSM reverse proxy in the middle. This might involve configuring Pomerium's Autocert feature.

Alternatively, mention a workaround for increasing nginx's proxy buffer size as described in pomerium/pomerium#4052 (comment).