Update understanding_metric_types.md

[andrewa-stripe]

1. Page markdown 'title' to match the link on the left (title). Not all pages do this, but most do and where they don't they're usually some extension of the link one the left (e.g. Visualization/Grafana -> "GRAFANA SUPPORT FOR PROMETHEUS")
2. Add link to the other page that documents the same information.

The two pages perhaps could be combined and conflated, but as a less review-heavy discoverability step, just linking from the later to the former.

Chose to link from this to the other, due to the other being more recently updated.

Specifically discovered this problem given this page did not link to the "choosing Summaries vs Histograms" page

Signed-off-by: andrewa-stripe [email protected]

I believe this falls under prometheus for the "Which Maintainers", so @roidelapluie & @LeviHarrison

[andrewa-stripe]

@roidelapluie & @LeviHarrison