Create documentation for Grafana server-rendered image embeds

Problem to solve

In this comment (https://gitlab.com/gitlab-org/gitlab-ee/issues/13039#note_206807570) it was discussed to add documentation for embedding Grafana images in issues.

Further details

Docs sections affected: https://docs.gitlab.com/ee/user/project/integrations/prometheus.html

Audience: Incident response - both internal to GitLab and external.

Proposal

Add documentation about the combination of two existing features:

• https://grafana.com/docs/reference/sharing/#direct-link-rendered-image
• https://docs.gitlab.com/ee/user/markdown.html#inline-html

Who can address the issue

Anyone with GitLab Flavoured Markdown experience.

Other links/references

Parent issue: https://gitlab.com/gitlab-org/gitlab-ee/issues/13039