cmd/derper: link to various derper docs in more places

In hopes it'll be found more.

Updates tailscale/corp#20844

Change-Id: Ic92ee9908f45b88f8770de285f838333f9467465
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f