Add layout nesting tip to the SEO chapters (#93)

* Add layout nesting tip to the SEO chapters

It can be confusing to people that once they set the `meta` override to the `app/routes/users+/$username.tsx` file, the pages in the child URL seem to ignore those overrides.

This PR adds some notes on the solution page of the "Meta Overrides" activity to refresh that information, already shown in the "Routes" part.

* Update exercises/06.seo/02.solution.nested/README.mdx

---------

Co-authored-by: Kent C. Dodds <[email protected]>

Author: julianobrasil

exercises/06.seo/02.solution.nested/README.mdx

@@ -4,3 +4,24 @@
 
 👨‍💼 Great! Now we can customize the meta tags on our routes! Let's do a bit more
 for our profile pages.
+
+<callout-info>
+	🦉 You'll notice that if you navigate to the `/users/kody` route, you'll see
+	the `Profile | Epic Notes` in the title. But you'll see only `Epic Notes`
+	when you navigate to one of its child URLs, like `/users/kody/notes`. It
+	behaves like that because, regardless the 2 URLs have a parent-child
+	relationship, in terms of layout, `/users/kody/notes` is not a child of 
+	`/users/kody`. Remember that the latter folder path is defined like this:
+	`routes/users+/$username.tsx` while the former's path is
+	`routes/users+/$username_+/notes.tsx`. That little underscore in the name of
+	the folder `$username_+` means that the pages inside will skip using the
+	layout defined in the `routes/users+/$username.tsx` file. In other words,
+	they won't be "layout-nested", even if they will be "url-nested". And, because
+	they won't be layout-nested, they won't inherit the meta override defined
+	in the `routes/users+/$username.tsx`, following, instead, the meta override
+	defined in the `routes/root.tsx` which is its parent regarding layout. One way
+	of seeing the layout parent-child relationships is by typing the 
+	`npx remix routes` in a terminal at the root of a remix project to see how
+	the routes will be configured. It should give you a better view of the
+	router layout-nesting structure.
+</callout-info>