Add missing Prometheus metrics image for documentation home page (opensearch-project#264)

The documentation home page references /images/dashboards/prometheus.png in
both Features.astro and CompetitorComparison.astro, but the file did not exist
in docs/public/images/dashboards/. This caused broken images on the landing
page at observability.opensearch.org.

Added the image using the metrics explore screenshot from the v3.6.0-alpha.1
release, resized to 1920px width for web use.

Resolves opensearch-project#263

Signed-off-by: joshuali925-osdbot <joshuali925-osdbot@users.noreply.github.com>
Signed-off-by: joshuali925-osdbot <278181255+joshuali925-osdbot@users.noreply.github.com>
Co-authored-by: joshuali925-osdbot <278181255+joshuali925-osdbot@users.noreply.github.com>

Author: joshuali925-osdbot