[improve-docs] Add taglines to articles

doc/articles/custom-installers.md

@@ -1,5 +1,5 @@
 <!--
-    tagline: Custom installers can be used to modify the way some packages are installed
+    tagline: Modify the way certain types of packages are installed
 -->
 # Setting up and using custom installers
 

doc/articles/handling-private-packages-with-satis.md

@@ -1,3 +1,6 @@
+<!--
+    tagline: Host your own composer repository
+-->
 # Handling private packages with Satis
 
 Satis can be used to host the metadata of your company's private packages, or

doc/articles/vendor-bins.md

@@ -1,3 +1,6 @@
+<!--
+    tagline: Expose command-line scripts from packages
+-->
 # bin and vendor/bin
 
 ## What is a bin?
@@ -99,4 +102,4 @@ An example of the former looks like this:
 
 Running `composer install` for this `composer.json` will result in
 all of the vendor bins being installed in `scripts/` instead of
-`vendor/bin/`.
+`vendor/bin/`.