add links for exporters in appendix

### Priority A

 * **node exporter**: `up`, disk usage, RAID, DRBD

 * **blackbox**: SSH, SMTP, HTTP(S) latency checks, Redis liveness

 * **textfile**: needrestart

 * **cert exporter**: cert expiry for private CA and LE certs

 * **barman exporter**: PostgreSQL backups validity

 * **bacula exporter**

 * **[blackbox][]**: SSH, SMTP, HTTP(S) latency checks, Redis liveness

 * **[textfile][]**: needrestart (see above)

 * **[cert-exporter][]**: cert expiry for private CA and LE certs

 * **[barman-exporter][]**: PostgreSQL backups validity

 * **[bacula-exporter][]**

[blackbox]: https://github.com/prometheus/blackbox_exporter/

[textfile]: https://github.com/prometheus/node_exporter#textfile-collector

### Priority B