Remove numbers from docs slugs (#293)

Co-authored-by: Lars Hvam <larshp@hotmail.com>

docs/config.json

@@ -2,35 +2,35 @@
   "docs": [
     {
       "uuid": "21b340f7-5709-4bfb-9e6f-b70e452c0d76",
-      "slug": "20-installation",
+      "slug": "installation",
       "path": "docs/INSTALLATION.md",
       "title": "Installing ABAP locally",
       "blurb": "Learn how to install ABAP locally to solve Exercism's exercises on your own machine"
     },
     {
       "uuid": "21a13a0d-687b-40d2-933e-d510241e10d8",
-      "slug": "10-learning",
+      "slug": "learning",
       "path": "docs/LEARNING.md",
       "title": "How to learn ABAP",
       "blurb": "An overview of how to get started from scratch with ABAP"
     },
     {
       "uuid": "e7aa0a32-7ee1-4a39-8f48-8f895ba2a4e9",
-      "slug": "30-tests",
+      "slug": "tests",
       "path": "docs/TESTS.md",
       "title": "Testing on the ABAP track",
       "blurb": "Learn how to test your ABAP exercises on Exercism"
     },
     {
       "uuid": "28d04b1a-195d-4633-9fb4-042f703a7d37",
-      "slug": "40-clone",
+      "slug": "clone",
       "path": "docs/CLONE.md",
       "title": "Clone the ABAP track",
       "blurb": "Instructions on how to clone the ABAP track to your own system"
     },
     {
       "uuid": "81756fdb-cbf1-4dd2-966f-59b820c447af",
-      "slug": "50-resources",
+      "slug": "resources",
       "path": "docs/RESOURCES.md",
       "title": "Useful ABAP resources",
       "blurb": "A collection of useful resources to help you master ABAP"