add notice for default behavior of using 3GB RAM

Clarify replace relabel behavior with non-matching regex

Fix blog sidenav.

This got broken in https://github.com/prometheus/docs/pull/365

Improve search engine result presentation

- best to keep title length under 55 characters
- descrioption should not be longer than 150-160 characters

Link to community section

Collapse inactive navigation menus

Use javascript to hide all inactive menu items and only show one active
menu item at a time.

Set descriptive front page title

The title of the homepage is used in Google search results.

document pushover configuration

fixes #345

Update link to Michael Stapelberg's German introduction talk

He gave a more up-to-date version of his previous talk at Easterhegg
2016. I think it makes more sense to link the newer talk here instead
of the old one.

Add docs for Nerve SD.

Fixes #298

Add GitHub star button to header.

Add a link to nginx-lua-prometheus

Add life360 logo to blog post

Add user interview with Life360

Mention CPU cores in peak performance benchmark.

Add PromCon link to landing page jumbotron.

Add rTorrent third party exporter

Fix wrong exposition format example

And also use seconds instead of milliseconds.