* added an boundary intro page * Update what-is-boundary.mdx deleted an extra space * added an image * Apply suggestions from Robin's code review Co-authored-by: Robin Beck <stellarsquall@users.noreply.github.com> Co-authored-by: Pete Pacent <ppacent@hashicorp.com> Co-authored-by: Robin Beck <stellarsquall@users.noreply.github.com>