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.

Add InfluxDB exporter

Fix config key typo

Fixes prometheus/alertmanager#274

Add ruby link to pushgateway section

Fix Slack send_resolved default

Add Ubiquiti UniFi third party exporter

Fix typo in AM client docs