Add new page to documentation titled "Why use pgroll?" (#806)

This PR adds a short page that calls out the benefits of using pgroll.
It is the part of the new, quicker onboarding experience in the
documenation.

---------

Co-authored-by: Andrew Farries <[email protected]>

Author: kvch

docs/config.json

@@ -5,6 +5,11 @@
       "href": "/getting-started",
       "file": "docs/getting-started.md"
     },
+    {
+      "title": "Why use pgroll",
+      "href": "/why-use-pgroll",
+      "file": "docs/why-use-pgroll.md"
+    },
     {
       "title": "Installation",
       "href": "/installation",

docs/why-use-pgroll.md

@@ -0,0 +1,15 @@
+# Why use pgroll?
+
+[pgroll](https://pgroll.com/) is a schema migration tool for Postgres. It is designed for application developers working on applications that require frequent schema changes but also need to maintain zero downtime around those schema changes. `pgroll` takes a different approach compared to most other migration tools on the market.
+
+There are two aspects that characterize `pgroll`'s approach to migrations.
+
+## Multi-version migrations
+
+Making a schema change with `pgroll` results in two versions of the schema; the one before the change and the one after the change - this allows applications to select which version of the schema they want to work with and allows side-by-side rollout of applications that require the new schema changes with old applications that may be incompatible with it.
+
+## Lock-safe migrations
+
+Migrations using `pgroll` are expressed declaratively, rather than using SQL directly. This allows `pgroll` to implement the steps required to perform the schema change in a safe manner, ensuring that any locks required on the affected objects are held for the shortest possible time.
+
+If you want to avoid worrying about schema changes, install `pgroll` and create your next migration with us.