chore: update slugs of intro pages

To make sure that we don't get directory listing when not explicitly
specifying the ‹intro› page.

Signed-off-by: Matej Focko <mfocko@redhat.com>

ib002/00-intro-ib002.md

@@ -1,7 +1,7 @@
 ---
 id: ib002-intro
 title: Introduction
-slug: /intro
+slug: /
 ---
 
 In this part you can find „random“ additional materials I have written over the

ib015/00-intro-ib015.md

@@ -1,5 +1,5 @@
 ---
 id: ib015-intro
 title: Introduction
-slug: /intro
+slug: /
 ---

ib110/00-intro-ib110.md

@@ -1,5 +1,5 @@
 ---
 id: ib110-intro
 title: Introduction
-slug: /intro
+slug: /
 ---

pb071/00-intro-pb071.md

@@ -1,5 +1,5 @@
 ---
 id: pb071-intro
 title: Introduction
-slug: /intro
+slug: /
 ---

pb161/00-intro-pb161.md

@@ -1,5 +1,5 @@
 ---
 id: pb161-intro
 title: Introduction
-slug: /intro
+slug: /
 ---