Updated Getting Started content.

[bdaehlie]

The current Getting Started page is not nearly as helpful or clear as it could be. We should simplify it, aiming the content squarely at the kinds of people who might navigate to that page.

We should resist the temptation to over-complicate things. We don't need to address every edge case on this page, and we can just assume that anyone who has a chance of successfully getting and running an ACME client on their own server already has basic sysadmin skills.

Also, we're not going to do a better job of explaining how to use Certbot (or any other ACME client for that matter) than the Cerbot website does, so let's just send people there for instructions on operating a client.

I've taken a stab at new content here. I don't think we need to get this to an ideal state to merge it - we just need to feel confident that it's significantly better than what we already have.

[aarongable]

Generally LGTM, but I think even this first version should include two additional pieces of information:

1. Our directory URL should still be listed here. I'm pretty sure this is the only place we list it today. Unless that's changing, it needs to still be here.
2. One quick sentence, probably near the directory URL, saying something like "if you're setting up your own client, test it against our staging API first" with a link to the docs about staging.