docs(testimonials): add Mike Karthauser

src/content/testimonial/marcos-duran.json

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+{
+  "name": "Marcos Duran",
+  "tagline": "Senior Software Engineer",
+  "text": "<p>I am a big fan of your git approaches. I especially remember pairing with you and watching how many commands you run to solve many problems and how fast you were. It's a skill I believe not many have, particularly those who are used to working with a GUI like me, and personally I think it is quite valuable.</p>",
+  "image": "marcos-duran.jpg",
+  "url": null,
+  "types": ["daily"]
+}

src/content/testimonial/mike-karthauser.json

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+{
+  "name": "Mike Karthauser",
+  "tagline": "Senior Software Engineer",
+  "text": "<p>Oliver’s approach to testing is a continual reminder of his commitment to delivering high-quality, bug-free, software.</p>",
+  "image": "mike-karthauser.jpg",
+  "url": null
+}

src/content/testimonial/patty-ocallaghan.json

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+{
+  "name": "Patty O'Callaghan",
+  "tagline": "Tech Lead",
+  "text": "<p>Just wanted to say that your blog is amazing <3 I absolutely love it and usually share it with colleagues and some of the kids at my Code Club.</p><p>Thanks for contributing to the community with your amazing content! :)</p>",
+  "image": "patty-ocallaghan.jpg",
+  "url": null,
+  "types": ["daily"]
+}

src/content/testimonial/stephen-mulvihill.json

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+{
+  "name": "Stephen Mulvihill",
+  "tagline": "Solutions Architect",
+  "text": "<p>I like the \"$ git log -S\" and \"$ git log --grep\" commands, will definitely be using these, thanks!</p>",
+  "image": "stephen-mulvihill.jpg",
+  "url": null,
+  "types": ["daily"]
+}