Add folder level TOC and change the way TOC is configured

[jsonmaur]

For folder-level TOC, it will inherit the title and description from the child page's front matter.

[coveralls]

Pull Request Test Coverage Report for Build 203

• 17 of 18 (94.44%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+1.3%) to 58.333%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/core/socket.js	0	1	0.0%

Totals
Change from base Build 200:	1.3%
Covered Lines:	375
Relevant Lines:	663

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💛 - Coveralls