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.
- add reusable Link and Button MDX components
- convert source-verified page CTAs to explicit buttons
- match live-site prose link styling and CTA destinations
- document link, button, and extraction authoring rules