install docs: Move requirements tl;dr to requirements doc.

This further reduces the wall of text on the install instructions.
Simultaneously it lightens up the pressure on this summary to be quite
so terse; expand a couple of items into multiple bullets (yet with
fewer words!) for greater readability.

docs/production/install.md

@@ -4,17 +4,8 @@ Make sure you want to install a Zulip production server. If you'd
 instead like to test or develop a new feature, we recommend the
 [Zulip server development environment](../development/overview.html#requirements) instead.
 
-You will need an Ubuntu system that satisfies
+You'll need an Ubuntu system that satisfies
-[the installation requirements](../production/requirements.html).  In short,
+[the installation requirements](../production/requirements.html).
-you need:
-* A dedicated machine or VM.
-* A supported OS:
-  * Ubuntu 16.04 Xenial 64-bit
-  * Ubuntu 14.04 Trusty 64-bit (not recommended for new installations)
-* At least 2GB RAM and 10 GB disk space (4GB and 2 CPUs recommended for 100+ users).
-* A DNS name, an SSL certificate, and credentials for sending email.
-  For most users, you can just use our handy `--certbot` option to
-  generate the SSL certificate.
 
 ## Step 1: Download the latest release
 

docs/production/requirements.md

@@ -5,6 +5,22 @@ like, we recommend creating an account on the
 [Zulip community server](../contributing/chat-zulip-org.html), or creating a test
 organization on <https://zulipchat.com>.
 
+## Summary
+
+To run a Zulip server, you will need:
+* A dedicated machine or VM
+* A supported OS:
+  * Ubuntu 16.04 Xenial 64-bit
+  * Ubuntu 14.04 Trusty 64-bit (not recommended for new installations)
+* At least 2GB RAM, and 10GB disk space
+  * If you expect 100+ users: 4GB RAM, and 2 CPUs
+* A hostname in DNS
+* An SSL certificate; for most users, you can just use our handy
+  `--certbot` option to obtain one
+* Credentials for sending email
+
+For details on each of these requirements, see below.
+
 ## Server
 
 #### General