Option to split root TOC into multiple child TOC files

[rwrife]

Added an option to split the root TOC into multiple child TOC files for each child folder down to a certain depth in the directory structure. Where adding -m 0 indicates that there should be no child TOC files created and only one large TOC at the root (the default) should be generated, -m 1 indicates to go one-level deep in the directory traversal, creating a toc at the root level that has hrefs that point the child folders, which themselves contain a TOC with remaining child structure defined. This allows for small TOC file at the root with easier management of child TOCs on large projects.

[asishskaria]

it is an awesome feature I am waiting so that I onboard huge docfx projects with humongous toc files without killing my ux.

[mtirionMSFT]

@rwrife Agree with @Ellerbach , please also modify the README with this new option.

[rwrife]

I updated the readme.