fix(content): add center prop to MDX page buttons

Adds center to every <Button> in content MDX files that previously relied
on the removed .button-center global class. Also includes minor content
cleanups in the same files: eyebrow markup for section labels in
aba-therapy.mdx, and removal of extra blank lines between list items in
learner-social-club.mdx.

www/src/content/pages/en/careers.mdx

@@ -16,7 +16,7 @@ Apply to join the Arizona Institute for Autism (AIA) team today. We are on a mis
 
 Our talented and passionate employees are a critical piece to the high-quality Autism services we provide along with our commitment to behavioral health, school solutions, and community outreach.
 
-<Button href="https://www.indeed.com/cmp/Arizona-Institute-For-Autism/jobs" target="_blank">View Open Positions</Button>
+<Button href="https://www.indeed.com/cmp/Arizona-Institute-For-Autism/jobs" target="_blank" center>View Open Positions</Button>
 
 ### Our Benefits
 
@@ -108,7 +108,7 @@ Work someone you love. Begin your journey with Arizona's leading ABA Integrated
 
 ![logo-BACB](/assets/images/logo-BACB.png)
 
-<Button href="https://www.indeed.com/cmp/Arizona-Institute-For-Autism/jobs" target="_blank">View Open Positions</Button>
+<Button href="https://www.indeed.com/cmp/Arizona-Institute-For-Autism/jobs" target="_blank" center>View Open Positions</Button>
 
 ## Benefits of Working at Arizona Institute for Autism
 
@@ -170,4 +170,4 @@ Take a fresh and innovative approach as a member of the Corporate team. Take on
 
 Our specialty interns help shape the future of our center carefully curated clinical, operations, advocacy, people, and other departments.
 
-<Button href="https://www.indeed.com/cmp/Arizona-Institute-For-Autism/jobs" target="_blank">View Open Positions</Button>
+<Button href="https://www.indeed.com/cmp/Arizona-Institute-For-Autism/jobs" target="_blank" center>View Open Positions</Button>