Remove stale build artifact and template leftovers from the repo root (#108)

The root index.html is a committed copy of an old Next.js build of the
homepage - it links Get Started to github.com/microsoft/documentdb,
carries the pre-rewrite hero with no MongoDB compatibility mention, and
references /_next/ chunks that do not exist in the repo. It is not what
GitHub Pages serves (deployment uploads the freshly built ./out via
actions/deploy-pages), but it is the first thing contributors open, and
if the Pages source setting were ever switched back to deploy-from-
branch this outdated page would ship as the live homepage.

Also drop dko-page-review.md (an internal page-review working document)
and the create-next-app template SVGs in public/ (next.svg, vercel.svg,
replit.svg), none of which are referenced anywhere in the site.

Author: GuanzhouSong