docs(store): Refined step 6,7, and 8 of getting started guide to be m…

[ptouchton]

…ore detailed.

closes #1880

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/ngrx/platform/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• [x ] Documentation has been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[x ] Documentation content changes
[ ] Other... Please describe:

Closes #1880

Does this PR introduce a breaking change?

[ ] Yes
[ x] No

Other information

[ngrxbot]

Preview docs changes for 19a79d4 at https://previews.ngrx.io/pr2627-19a79d4c/

[timdeschryver]

LGTM, thanks @ptouchton
I do have one question about the changes to the examples.

[timdeschryver]

Awesome, thanks!