Add new Sitecore Send documentation to the portal

[jst-cyr]

There is new documentation pages for Sitecore Send: https://doc.sitecore.com/send/

Could we have these added (at least the top level)? This seems to be end user documentation, not developer documentation, but it would still be good to link to the category and hopefully at a later date there can be some specific developer docs we can link to.