Remove Jekyll configuration, layouts, and templates

Remove all Jekyll files now that the site is built with Vite.
Includes _config.yml, Gemfile, _layouts/, _includes/, and
root HTML templates.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Michael Garber <michael.garber@hashgraph.com>

Author: mgarbs