Website - user story image update (#13997)

- Updated images for the three user stories.
- Fixed a Markdown error in one of the Wayfair user story headings.

Closes https://github.com/fleetdm/fleet/issues/13227

# Checklist for submitter

- [x] Manual QA for all new/changed functionality

articles/fleet-user-stories-f100.md

@@ -2,7 +2,7 @@
 
 ## Cloud Security Technical Lead — F100 security and networking company.
 
-![Two people talking about Fleet](../website/assets/images/articles/fleet-user-stories-f100-cover-800x450@2x.jpeg)
+![Two people talking about Fleet](../website/assets/images/articles/fleet-user-stories-f100-cover-800x450@2x.png)
 
 When we spoke recently, our next Fleet user (whose name and employer must remain anonymous for contractual reasons) gave us candid insights into how osquery and Fleet has been adopted at their F100 security and networking company.
 
@@ -39,4 +39,4 @@ I don’t directly deal with that — that’s our security operations team, mos
 <meta name="authorFullName" value="Mike Thomas">
 <meta name="publishedOn" value="2021-09-29">
 <meta name="articleTitle" value="Fleet user stories — F100">
-<meta name="articleImageUrl" value="../website/assets/images/articles/fleet-user-stories-f100-cover-800x450@2x.jpeg">
+<meta name="articleImageUrl" value="../website/assets/images/articles/fleet-user-stories-f100-cover-800x450@2x.png">

articles/fleet-user-stories-schrodinger.md

@@ -2,7 +2,7 @@
 
 ## Jason Walton — Director of information security @ Schrödinger
 
-![Four speech bubbles and four stars emanating from the Fleet logo](../website/assets/images/articles/fleet-user-stories-schrodinger-cover-800x450@2x.jpeg)
+![Four speech bubbles and four stars emanating from the Fleet logo](../website/assets/images/articles/fleet-user-stories-schrodinger-cover-800x450@2x.png)
 
 Jason Walton gives us some insight into how his team uses Fleet and osquery at Schrödinger.
 
@@ -27,4 +27,4 @@ We actually don’t use a SIEM for this reason. We rely on alerts and signals fr
 <meta name="authorFullName" value="Mike Thomas">
 <meta name="publishedOn" value="2021-09-10">
 <meta name="articleTitle" value="Fleet user stories — Schrödinger">
-<meta name="articleImageUrl" value="../website/assets/images/articles/fleet-user-stories-schrodinger-cover-800x450@2x.jpeg">
+<meta name="articleImageUrl" value="../website/assets/images/articles/fleet-user-stories-schrodinger-cover-800x450@2x.png">

articles/fleet-user-stories-wayfair.md

@@ -2,7 +2,7 @@
 
 ## Ahmed Elshaer — DFIR, Blue Team, SecOps @ Wayfair
 
-![Two people talking about Fleet](../website/assets/images/articles/fleet-user-stories-wayfair-cover-800x450@2x.jpeg)
+![Two people talking about Fleet](../website/assets/images/articles/fleet-user-stories-wayfair-cover-800x450@2x.png)
 
 This week, I spoke with Ahmed Elshaer (DFIR, Blue Team, SecOps) about how Wayfair uses Fleet and osquery:
 
@@ -13,7 +13,8 @@ We were looking for a tool that provided linux logging, and incident response ca
 ### Why are you using Fleet?
 
 We have POC’d couple free options and Fleet was the highest engagement and continuous development although it may be missing some features.
-How do your end users feel about Fleet?
+
+### How do your end users feel about Fleet?
 
 We are using Fleet only in the remote query on scale, so we find Fleet is doing a good job in that area, and it’s easy to use for any new members.
 
@@ -26,4 +27,4 @@ We have lots of queries that generate logs, but the ones that go into alerts are
 <meta name="authorFullName" value="Mike Thomas">
 <meta name="publishedOn" value="2021-08-20">
 <meta name="articleTitle" value="Fleet user stories — Wayfair">
-<meta name="articleImageUrl" value="../website/assets/images/articles/fleet-user-stories-wayfair-cover-800x450@2x.jpeg">
+<meta name="articleImageUrl" value="../website/assets/images/articles/fleet-user-stories-wayfair-cover-800x450@2x.png">