docs(www): only enable scroll position on backward navigation

[timdeschryver]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/ngrx/platform/blob/main/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?

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

What is the current behavior?

The scroll position is always restored.
This is not very useful when you read a page, scroll to the bottom, navigate to another page.
I expect to scroll to the top, instead the scroll position remains the same (at the bottom of the apge)

What is the new behavior?

This change only restores the scroll position while navigating back.

Does this PR introduce a breaking change?

[ ] Yes
[ ] No

Other information

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