Closes: tpo/web/donate-neo#75

I've just tried this this morning and it works as expected: thunderbird
presents the list of calendars with just the missing ones selected.

I was doing this to bypass my spellchecker, but that's dumb, added it
to a file-local list instead.

The service had stopped working for me since it did not have access to
the environment variable and thus was not reaching out to my yubi key.

We have some of those details in the `blackbox_exporter` section and
really it's related to how scrape targets are defined.

Also, to make the section more digestible, the section about configuring
blackbox targets is now split between scrape jobs vs targets and a bit
more context is given about the two.

This was already documented, but buried in the puppet section.

/cc @hiro

review privacy policy (team#41727)

See merge request !59