Update ops docs: No need for manually creating TLS keys for the payment server anymore

See merge request !212

Since !146 (merged on 2021-09-07) we let the NixOS Let's Encrypt integration
with NGINX handle TLS.  That integration cares for interim self-signed certs
until real ones are aquired.  The section about creating TLS certs by hand so
the service will start is obsolote.

words about recovering from backup

Closes privatestorageops#276

See merge request !206

Don't literalExample all the examples

See merge request !209

literalExample is deprecated and overused. See:
https://whetstone.privatestorage.io/privatestorage/PrivateStorageio/-/merge_requests/201#note_16966

Monitor TLS cert expiry

Closes privatestorageops#363

See merge request !201

Thanks for the suggestion @jcalderone!

As per our discussion in PrivateStorage sync meeting today.

Update nixpkgs.

See merge request !207

Update zkapauthorize to use mach-nix and nixos-21.05.

See merge request !166

Weekly nixpkgs updateUpdate nixpkgs.

See merge request !205

Weekly nixpkgs update.

See merge request !204

Fix dev docs for repo update script

See merge request !203

Thanks @jcalderone for pointing me to it

Response time, probe fails and TLS expiry incl. alerts.

The Blackbox exporter can be used to check whether some services do answer;
We'll use it for our HTTPS endpoints.
Especially handy is it's checking for TLS cert expiry.

Alerting: Only alert when an exporter is not scrapable for 5 mins or longer

See merge request !200

This change reflects my intention of having an alert fire when a node
is not scrapable for five minutes or longer.  I hadn't understood the
default avg() function well enough before, yielding an alert that would
fire too soon (i.e. any time a node was down, even only one minute for a
reboot).

Use `$__file{}` for grafana's slack URL secret.

See merge request !191

Weekly nixpkgs update.

See merge request !199

Monitoring: Add dashboard and alerting for used swap

Closes privatestorageops#358

See merge request !198

Alerting: Don't when no scraping error ever happened.

See merge request !197

Use a version of vulnix that doesn't collapse derivations with different sets of patches.

See merge request !196

Update PaymentServer.

See merge request !195

Expose zkap-spending-service metrics.

See merge request !184