fix: expand custom/genomic surveillance tutorial intros

docs/src/tutorial/custom-data.rst

@@ -1,7 +1,7 @@
 Run the workflow using custom data
 ==================================
 
-In this tutorial, you will run the workflow using custom focal data in addition to the example reference data. The reference data will serve as background context for the new data.
+This tutorial builds on the previous tutorial. Here, we will walk through how to supply your own genomic data and analyze this with the example reference data, which will serve as background context for the new data. We will explain how to get the new data from GISAID, but you could replace this data with your own private sequences if needed.
 
 .. contents:: Table of Contents
    :local:

docs/src/tutorial/genomic-surveillance.rst

@@ -1,7 +1,7 @@
 Run the workflow using a genomic surveillance configuration
 ===========================================================
 
-In this tutorial, you will replace the custom data with meaningful focal data and use a configuration commonly found in genomic surveillance applications.
+In the previous tutorial we showed how to analyze a small set of GISAID ("custom") data in the context of a small set of reference data. For genomic surveillance applications, we often have a set of data specific to our question of interest, for instance a set of sequences from a particular geographic area, which is referred to as the focal set. We want to analyze this focal set in a global context, and since there are millions of global sequences we want to subset these based on genomic proximity to our focal set.
 
 .. contents:: Table of Contents
    :local: