Change introduction doc grouping (#4583)

The introduction doc grouping reads as a step-by-step tutorial for creating the `friends` example.

The Embedded Schema docs being included can be confusing for someone using the bottom nav, since they would get moved from the friends example to an unrelated user profile example. It seems to be more fitting as a how-to.

I also moved out the testing docs for similar reasons. In Phoenix, testing has its own section, so I did the same for Ecto (since the testing documentation is disconnected from `friends`.

Author: TylerWitt

guides/introduction/Embedded Schemas.md guides/howtos/Embedded Schemas.md

@@ -142,21 +142,21 @@ defmodule Ecto.MixProject do
   def extras() do
     [
       "guides/introduction/Getting Started.md",
-      "guides/introduction/Embedded Schemas.md",
-      "guides/introduction/Testing with Ecto.md",
       "guides/howtos/Aggregates and subqueries.md",
       "guides/howtos/Composable transactions with Multi.md",
       "guides/howtos/Constraints and Upserts.md",
       "guides/howtos/Data mapping and validation.md",
       "guides/howtos/Duration Types with Postgrex.md",
       "guides/howtos/Dynamic queries.md",
+      "guides/howtos/Embedded Schemas.md",
       "guides/howtos/Multi tenancy with query prefixes.md",
       "guides/howtos/Multi tenancy with foreign keys.md",
       "guides/howtos/Self-referencing many to many.md",
       "guides/howtos/Polymorphic associations with many to many.md",
       "guides/howtos/Replicas and dynamic repositories.md",
       "guides/howtos/Schemaless queries.md",
       "guides/howtos/Test factories.md",
+      "guides/testing/Testing with Ecto.md",
       "guides/cheatsheets/crud.cheatmd",
       "guides/cheatsheets/associations.cheatmd",
       "CHANGELOG.md"
@@ -169,7 +169,8 @@ defmodule Ecto.MixProject do
     [
       Introduction: ~r/guides\/introduction\/.?/,
       Cheatsheets: ~r/cheatsheets\/.?/,
-      "How-To's": ~r/guides\/howtos\/.?/
+      "How-To's": ~r/guides\/howtos\/.?/,
+      Testing: ~r/testing\/.?/
     ]
   end