Rearrange docs table of contents for coherence.

The order and structure of the Read The Docs documentation
was fairly arbitrary; here's a suggestion for how to change
its information architecture so it's more useful to a new
contributor. This adds a short Zulip overview, and a table
of contents and code contribution intro for new contributors.

Fixes: zulip#668.

Signed-off-by: Sumana Harihareswara <sumanah@panix.com>

docs/code-contribution-checklist.rst

@@ -0,0 +1,23 @@
+=======================
+Code contribution guide
+=======================
+
+Thanks for contributing to Zulip! We hope this guide helps make the
+process smooth sailing for you.
+
+The "Ways to Contribute" and "How to get involved" sections of
+:doc:`readme-symlink` are a good place to start! After that, check out
+the tips below.
+
+
+.. toctree::
+   :maxdepth: 2
+
+   code-style
+   markdown
+   testing
+   logging
+   mypy
+   front-end-build-process
+   queuing
+   schema-migrations

docs/index.rst

@@ -6,22 +6,31 @@
 Welcome to Zulip documentation!
 ===============================
 
+Zulip is a powerful, open source group chat application. Written in
+Python and using the Django framework, Zulip supports both private
+messaging and group chats via conversation streams.
+
+Zulip also supports fast search, drag-and-drop file uploads, image
+previews, group private messages, audible notifications, missed-message
+emails, desktop apps, and much more.
+
+Further information on the Zulip project and its features can be found
+at `https://www.zulip.org <https://www.zulip.org>`__ and in these
+docs.  Our code is available at `our GitHub repository
+<https://github.com/zulip/>`__.
+
+This set of documents covers installation and contribution instructions.
+
 Contents:
 
 .. toctree::
-   :maxdepth: 2
+   :maxdepth: 3
 
+   readme-symlink
    integration-guide
    new-feature-tutorial
+   code-contribution-checklist
    directory-structure
-   code-style
-   testing
-   logging
-   markdown
-   queuing
-   schema-migrations
-   front-end-build-process
-   mypy
    translating
    changelog
    roadmap

docs/readme-symlink.md

@@ -0,0 +1 @@
+../README.md