Versioning alertmanager docs

[Julien Pivotto]

Hi there,

Since this is cross repositories I am posting a message here.

There are two pull request in progress that are linked together somehow.
However, they should be merged in a specific order.

https://github.com/prometheus/alertmanager/pull/2292

This pull request moves the alertmanager doc to its repository, in the
0.20 branch.

Such an attempt was made in the past but it was never finalized. I made
some optimization to the build system so that we can do it without
reducing the number of prometheus releases docs we publish.

Once prometheus/alertmanager/pull/2292 is merged, I would reconfigure
the following PR to point to the alertmanager repo instead of my fork:

https://github.com/prometheus/docs/pull/1657

Then, I would craft a prometheus/alertmanager PR from release-0.20 to
the release-0.21 branch of alertmanager with additional commits:

- the WebHook changes
- the removal of HipChat

We would then add release-0.21 in the prometheus/docs repository, like
we do for prometheus releases.

Then, release-0.21 branch would be merged to prometheus/alertmanager
master and we would then create another pull request which would cherry
pick: https://github.com/prometheus/docs/pull/1663

Regards,

--
Julien Pivotto
@roidelapluie