Support of "sidebarDepth: 3"

[YGYOOO]

Feature request

Now the max value of sidebarDepth is 2, could you support 3?

What problem does this feature solve?

In some cases, only 2 levels may not meets the needs.

What does the proposed API look like?

sidebarDepth: 3

How should this be implemented in your opinion?

I think extracting h4 would be similar with extracting h2 and h3. It may not be hard to be implemented.

Are you willing to work on this yourself?

Currently not.

[ulivz]

Duplicate of #984

[kshefchek]

According to the docs this has not been added, and is not covered by #984 or PR #1945, please consider reopening

EDIT: It does seem with setting:
extractHeaders: [ 'h2', 'h3', 'h4' ]

And
sidebarDepth: 2,
the h4 headers are included in the sidebar