[Feature Request] Add options to disable `wordCount` feature

**Is your feature request related to a problem? Please describe.**

Currently docfx calculate `wordCount` for conceptual files.
And `wordCount` is stored in `viewModel` but is not rendered to HTML (by default).

There are some custom templates that use this `wordCount` to display `Time to read`(> 1 minute to read) information.
(e.g. https://docs.dnncommunity.org/content/getting-started)

But `wordCount` is not used most use case.
So I want to disable this feature on my environment.

**Describe the solution you'd like**

1. Add options to disable `wordCount` feature.
**or**
2. Disable `wordCount` feature by default (Breaking Changes).
And add options to enable feature who want to `wordCount` feature.