Clarify that the in_cluster option only affects discovery.

It wasn't clear to me that the `in_cluster` option for a
`kubernetes_sd_config` only affected authentication for service
discovery, and that to actually scrape the targets, it was still
necessary to explicitly set authentication options like `tls_config` and
`bearer_token_file`, for example, when scraping the Kubernetes
apiserver.

This patch clarifies that in the documentation for `in_cluster`, and
adds a link to the example Prometheus configuration for Kubernetes in
the Prometheus repo that explained this distinction better, and also
provides a useful real-world example of how to monitor a Kubernetes
cluster with Prometheus.

Add BOSH exporter to third-party exporters

Document minute()

optimize png images using zopflipng

Add GCE SD docs

Clarify that rule files are configured via globs

Fix download page problems

Prometheus server 1.2 is out!

Introduce categories to exporterlist

As @juliusv mentioned once in #prometheus, the list of external exporters has grown quite a bit. This attempts to increase overview by introducing categories for the third party exporters.

The chosen categories are a first proposal and might still be subject to change, depending on the feedback, any of which is greatly appreciated!

This reverts commit c0f332ee.

Upon request by Puck` on IRC.

Document alert_relabel_configs.

Fixes #568

Add base units requirement to naming best practices doc

Add prometheus-users@ to Community page

Fix typos across the docs repo.

Fixed some blog and documentation typos.

Add Incopro interview

Added ProxySQL exporter to the list.

Add information on using pre-built / contributed dashboards.

Fix `label_replace` documentation

Updated (typos and copy clarity etc.) following PR comments.

With the fix in place, the documentation is in line with the intention
expressed by tests,
e.g. https://github.com/prometheus/prometheus/blob/master/promql/testdata/functions.test#L173-L176
and
https://github.com/prometheus/prometheus/blob/master/promql/testdata/functions.test#L195-L199
.

querying examples: fix proctype vs proc metric key

Add ec2_instance_state