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

Add Jenkins exporter

Exposition formats: Clarify edge cases about sorting and uniqueness

Add a few helpful links to the media section

Also, update FAQs a bit. (Needs a more thorough update, though.)

Also, restructure a bit.

Add PowerDNS exporter

Add initial glossary

Add Ceph exporter to list of 3rd party exporters

fix typo

add nsq-exporter

operators.md: Switch order in operator precedence list

functions.md: Add cross-references to operators.md

Document the scrape_duration_seconds timeseries