Add a partners showcase

[indirect]

We're gonna have partners! They will (we expect) break down into two categories:

1. Partners who build things using Ruby (agencies, consultancies, freelancers, etc).
2. Partners who hire Ruby devs (job sites, individual companies that are members).

We should build a page to showcase those partners, helping places looking for Ruby devs find agencies, and helping devs looking for jobs to find work options.