fix($default-theme): Fix deep sidebar links rendenring

[kefranabg]

Summary

I realized deep sidebar links rendering was not working as expected.

Before fix:

After fix:

The sidebar links rendering code seems a bit complex, I'll try to refactor to make it more readable later.

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Build-related changes
• Other, please describe:

If changing the UI of default theme, please provide the before/after screenshot:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)

You have tested in the following browsers: (Providing a detailed version will be better.)

• Chrome
• Firefox
• Safari
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• IE

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature
• Related documents have been updated
• Related tests have been updated

To avoid wasting your time, it's best to open a feature request issue first and wait for approval before working on it.

Other information:

[meteorlxy]

Let's merge this PR first, as the sidebar of our docs has been broken for days. cc @ulivz

Looking forward to the further refactor @kefranabg ❤️

[vue-bot]

Hey @kefranabg, thank you for your time and effort spent on this PR, contributions like yours help make Vue better for everyone. Cheers! 💚