Render tarball sha256 checksum if available for release

This is for the 1.6 release of the Prometheus server.

Document that target_label can do regex replacement

Document scrape_samples_post_metric_relabeling and scrape_samples_scraped

Mention timestamped samples cannot yet be scraped

This is just swapping two paragraphs and moves the explanation of the
example after the example itself.

Update storage documentation for the Prometheus 1.6 release

Fixes #602

Fixes #614
Fixes #679

Added new company logos

Update FAQ

This is a fall-out of updating the documentation of 1.6.

However, all the updates here are not 1.6-specific, so I want to merge
them into master directly. (Which simplifies things as there are some
FAQ updates in master already that are not in the next-release branch
yet.)

blog: Announce 2.0 alpha release

Pitch CloudNativeCon Prometheus videos on front page

It has unintentionally become a defacto listing of exporters, some of whom
we may not wish to list here for various reasons or for which noone has
sent a PR yet. This helps make it easier for users to find exporters,
and potentially aid in development of the ones that are still maturing.

Add remote_read docs and update remote_write ones

Next step: add example code for remote read, point more precisely to
examples and specs for each.

FlowDock integration is not currently supported in .5 version of AM.

See https://github.com/prometheus/alertmanager/issues/236

Installation: mention Puppet module

The puppet prometheus module works fine with the current version of prometheus.

Document custom query timeouts.

Remove remaining references to PromDash