docs: Improve installation instruction headings.

The previous RTD headings were unclear and resulted in some users
attempting to install the Zulip production environment for doing
development.  This clarifies the situation.

docs/index.rst

@@ -49,7 +49,7 @@ Contents:
    :caption: Zulip in production
 
    prod-requirements
-   prod-install
+   Installing a production server <prod-install>
    prod-troubleshooting
    prod-customize
    prod-maintain-secure-upgrade
@@ -60,9 +60,9 @@ Contents:
 
 .. toctree::
    :maxdepth: 2
-   :caption: Installation for developers
+   :caption: Development environment
 
-   Overview <dev-overview>
+   Development environment installation <dev-overview>
    Recommended setup (Vagrant) <dev-env-first-time-contributors>
    Advanced setup (non-Vagrant) <dev-setup-non-vagrant>
    Using the development environment <using-dev-environment>

docs/prod-install.md

@@ -1,11 +1,15 @@
-# Installation
+# Production Installation
 
-Ensure you have an Ubuntu system that satisfies [the installation
-requirements](prod-requirements.html).  In short, you should have an
-Ubuntu 14.04 Trusty or Ubuntu 16.04 Xenial 64-bit server instance,
-with at least 4GB RAM, 2 CPUs, and 10 GB disk space.  You should also
-have a domain name available and have updated its DNS record to point
-to the server.
+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](dev-overview.html#requirements) instead.
+
+You will need an Ubuntu system that satisfies
+[the installation requirements](prod-requirements.html).  In short,
+you should have an Ubuntu 14.04 Trusty or Ubuntu 16.04 Xenial 64-bit
+server instance, with at least 4GB RAM, 2 CPUs, and 10 GB disk space.
+You should also have a domain name available and have updated its DNS
+record to point to your server.
 
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