Add more steps to the play with grafana mimir page

[davecb]

I've fleshed out the steps slightly, as this is my (and others') first time through, and one doesn't often know what is needed.

What this PR does

Adds a clarification to instructions which can be unclear the first time one does it.
As this is a demonstration recommended to new adopters, a lot of first-time users
will be trying it.

Which issue(s) this PR fixes or relates to

/docs/sources/mimir/get-started/play-with-grafana-mimir/index.md #7844 in which I
failed to guess the right option, and had to ask

Fixes #7844

Checklist

• Tests updated.
• [ x ] Documentation added.
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX].
• about-versioning.md updated with experimental features.

[CLAassistant]

All committers have signed the CLA.

[davecb]

There are other issues in the "play with grafana" page.
For example, the section on doing a reporting rule is (nearly?) identical to the section on alert rules

[jhesketh]

Thanks so much for the fixes :-).

[jdbaldry]

Unrelated to your change but since we are here, it may be nice to get the style in-keeping with Writers' Toolkit guidelines.

• bold is used for UI elements: https://grafana.com/docs/writers-toolkit/write/style-guide/style-conventions/#bold.

• We can make use of the Markdown renderer knowing how to auto-increment list numbers and just 1. as the number for each step: https://grafana.com/docs/writers-toolkit/write/markdown-guide/#numbered-lists

[jdbaldry]

docs changes look good from a style perspective