[Docs] Document our numeral-js fork in the Kibana docs

[wylieconlon]

As part of our effort to move away from numeral, we should stop relying on the third-party docs. One option is to set up an external docs page on github pages, but it seems easier to add a page to the Kibana documentation.

I plan on writing this docs page because I recently added a new bytes format.

[elasticmachine]

Pinging @elastic/kibana-docs (Team:Docs)