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

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