Exporters page: Add WebDriver exporter

Clarify that `deriv` is per-second.

functions: Clarify over-time semantics

Update FAQ on dashboards to reflect latest advice

Minor spelling fix for directly instrumented software

* Use d3 to handle keyboard event

jQuery isn't always being loaded before the
routing-tree script is executed.

We cannot guarantee when jQuery will be available,
but d3 will be loaded before the routing-tree
script. This change preserves the previous
functionality.

* Add simple case for handling fewer than 3 nodes

The tree algorithm being used doesn't properly handle 1 or 2 nodes.

Deleted leftover "e.g."

[docs] fix relabel default configs

operating/configuration: Remove kubernetes_sd_config kubelet_port

According commit [206bcfcd](https://github.com/prometheus/prometheus/commit/206bcfcdaac356e8bdb0cd91f3478f111eaa6d30)
kubeletPort config option was removed (release 0.20.0)

docs/alerting: update configuration docs

Merge next release doc into master

configuration: Update scrape config

Add missing underscore to webhook_config variable

fixed a typo

added alertmanager start example

with example config.file argument

Update kubernetes SD documentation

Update configuration documentation to present correct information about port/container selection algorithms; and to provide a warning about the requirement to expose at least one port on pods

Fix `post` values in result

Update docs for server_name in <tls_config>

Improve k8 discovery docs.

Fix formatting.
Remove long example from reference documentation, point off
to existing example instead.

Remove digitalocean/rsyslog_exporter from lost of exporters.