docs(store): add example of createFeature extraSelectors

[Armenvardanyan95]

closes #3775

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)
• Documentation has been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

Documentation and examples have been added to the "Feature Creator" section of the "Advanced" documentation

[ ] 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 #3775

Does this PR introduce a breaking change?

[ ] Yes
[x] No

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[timdeschryver]

Thanks Armen!

[markostanimirovic]

LGTM 👍 Just one minor suggestion, so that the code formatting is consistent on this page:

[Armenvardanyan95]

Thanks, @markostanimirovic, I added the suggestion

[timdeschryver]

Thanks!
Feel free to open a PR/issue for the entity+createFeature recipe.