fix(docsWeb): change routerLink to link on app-button in header

[cmgriffing]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/scullyio/scully/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have 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)
• Other... Please describe:

What is the current behavior?

In the Header, the current docs site shows undefined in the path of the href. This breaks opening main menu links in a new tab.

Issue Number: N/A

What is the new behavior?

The routerLink input is fixed to be link as expected by the app-button component.

Does this PR introduce a breaking change?

• Yes
• No

Other information