feat: Alpha landing page (#478)

* refactor: allow per-page layout

* refactor: type blog page `getStaticProps`

* refactor: delete obsolete Campaign page layout  condition

* feat: letter v1

* feat: make RSVP sticky in mobile

* style: fix letterContainer height

* feat: update page meta tags

* feat: apply hidden mask to md letter

* feat: apply hidden mask to xs letter

* fix: undo pages refactor

* feat: apply hidden mask to sm letter

* fix: have a different wrapper for the sticky div

* chore: rm unused asset

* chore: update CTA links

* fix: make visibility icon fixed

* fix: use Links for the CTA

* feat: pass an initial value to useScrollReveal