Document complement set operator "unless"

Add documentation about selection of chunk encoding

Document without aggregation modifier

Rework storage documentation and add "rushed mode"

Since this also includes the information of suspended ingestion upon
too many chunks in memory, it can only be merged once this has been
implemented (I'm working on it next).

This closes #267

Update architecture diagram

Include Python pushgateway in Pushing docs

Currently only shell, Java and Go are listed. Python supports pushgateways as well but requires users to go to the client library page to discover about it.

Blog post about 1-year announcement anniversary.

Make logos smaller and responsive.

Remove unused image.

Add company logos to the frontpage.

Since it's plural now, it's no longer a placeholder.

Use light color in code blocks

The current black code blocks are very dominant. In my personal view
they are too disruptive when scaning a page. This change makes them
similar to inline code examples.

Table of contents

* Remove duplicated list of metric types
* Consolidate function titles
* Remove too wide table in functions section
* Shorten configuration headings

Fix 2 typos

Add a link to the PgBouncer exporter

Add pointer to exporter guidelines