feat(theme-default)!: rename `auto` to `heading` in sidebar and improve layout types

[Mister-Hope]

This PR improves types of navbar and sidebar by making them stricter and more accurate.

E.g.: When users are nesting navbar to 3 levels, type issue is thrown.

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Also, it changes auto keyword to heading, so that structure keyword can be introduced later.

[coveralls]

Pull Request Test Coverage Report for Build 9317078044

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 56.545%

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Totals
Change from base Build 9302195921:	0.0%
Covered Lines:	1010
Relevant Lines:	1600

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💛 - Coveralls