Add generator video to Quick Start guide

What does this MR do and why?

It adds a video walk through of how to use the generator to create events and metrics.

In addition, it makes some small improvements to the generator example.

Before

bin/rails g gitlab:analytics:internal_events \
--time_frames=7d 28d \
--group=project_management \
--stage=plan --section=dev \
--event=project_created \
--unique=user.id \
--mr=https://gitlab.com/gitlab-org/gitlab/-/merge_requests/121544

After

bundle exec rails generate gitlab:analytics:internal_events \
--time_frames=7d 28d \
--group=project_management \
--stage=plan \
--section=dev \
--event=project_created \
--unique=user.id \
--mr=https://gitlab.com/gitlab-org/gitlab/-/merge_requests/121544

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #422692 (closed)